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The Rule Interchange Format (RIF) is a format for interchanging rules over the Web. Rules that are exchanged using RIF may refer to external data sources and may be based on data models that are represented using a language different from RIF. The Resource Description Framework RDF [RDFConcepts] is a Webbased language for the representation and exchange of data; RDF Schema (RDFS) [RDFSchema] and the OWL Web Ontology Language [OWL2Syntax] are Webbased languages for representing and exchanging ontologies. This document specifies how combinations of RIF documents and RDF data and RDFS and OWL ontologies are interpreted; i.e., it specifies how RIF interoperates with RDF, RDFS, and OWL. We consider here OWL 2 [OWL2Syntax], which is an extension of OWL 1 [OWLReference]. Therefore, the notions defined in this document also apply to combinations of RIF documents with OWL 1 ontologies.
We consider here the RIF Basic Logic Dialect (BLD) [RIFBLD] and RIF Core [RIFCore], a subset of RIF BLD. The RIF Production Rule Dialect (PRD) [RIFPRD] is an extension of RIF Core. Interoperability between RIF and RDF/OWL is only defined for the Core subset of PRD. In the remainder, when speaking about RIF documents and rules, we refer to RIF Core and BLD.
RDF data and RDFS and OWL ontologies can be represented using RDF graphs. There exist several alternative syntaxes for OWL ontologies; however, for exchange purposes it is assumed they are represented using RDF graphs.
Several syntaxes have been proposed for the exchange of RDF graphs, the normative syntax being RDF/XML [RDFSyntax]. RIF does not provide a format for exchanging RDF graphs; it is assumed that RDF graphs are exchanged using RDF/XML, or any other syntax that can be used for representing or exchanging RDF graphs.
A typical scenario for the use of RIF with RDF/OWL is the exchange of rules that use RDF data and/or RDFS or OWL ontologies: an interchange partner A has a rules language that is RDF/OWLaware, i.e., it supports the use of RDF data, it uses an RDFS or OWL ontology, or it extends RDF(S)/OWL. A sends its rules using RIF, possibly with references to the appropriate RDF graph(s), to partner B. B receives the rules and retrieves the referenced RDF graph(s). The rules are translated to the internal rules language of B and are processed, together with the RDF graphs, using the RDF/OWLaware rule engine of B. The use case Vocabulary Mapping for Data Integration [RIFUCR] is an example of the interchange of RIF rules that use RDF data and RDFS ontologies.
A specialization of this scenario is the publication of RIF rules that refer to RDF graphs; publication is a special kind of interchange: one to many, rather than onetoone. When a rule publisher A publishes its rules on the Web, there may be several consumers that retrieve the RIF rules and RDF graphs from the Web, translate the RIF rules to their respective rules languages, and process them together with the RDF graphs in their own rules engines. The use case Publishing Rules for Interlinked Metadata [RIFUCR] illustrates the publication scenario.
Another specialization of the exchange scenario is the Interchange of Rule Extensions to OWL [RIFUCR]. The intention of the rule publisher in this scenario is to extend an OWL ontology with rules: interchange partner A has a rules language that extends OWL. A splits its ontology+rules description into a separate OWL ontology and a RIF document, publishes the OWL ontology, and sends (or publishes) the RIF document, which includes a reference to the OWL ontology. A consumer of the rules retrieves the OWL ontology and translates the ontology and document into a combined ontology+rules description in its own rule extension of OWL.
A RIF document that refers to (imports) RDF graphs and/or RDFS/OWL ontologies, or any use of a RIF document with RDF graphs, is viewed as a combination of a document and a number of graphs and ontologies. This document specifies how, in such a combination, the document and the graphs and ontologies interoperate in a technical sense, i.e., the conditions under which the combination is satisfiable (i.e., consistent), as well as the entailments (i.e., logical consequences) of the combination.
The interaction between RIF and RDF/OWL is realized by connecting the model theory of RIF [RIFBLD] with the model theories of RDF [RDFSemantics] and OWL [OWL2Semantics], respectively.
The notation of certain symbols in RIF, particularly IRIs and plain literals, is slightly different from the notation in RDF/OWL. These differences are illustrated in the Section Symbols in RIF Versus RDF/OWL.
The RDF semantics specification [RDFSemantics] defines four normative notions of entailment for RDF graphs: Simple, RDF, RDFS, and Datatype entailment. OWL 2 specifies two different semantics, with corresponding notions of entailment: the direct semantics [OWL2Semantics], which specifies OWL 2 DL entailment, and the RDFbased semantics [OWL2RDFBasedSemantics], which specifies OWL 2 Full entailment. This document specifies the interaction between RIF and RDF/OWL for all six notions. The Section RDF Compatibility is concerned with the combination of RIF and RDF/RDFS. The combination of RIF and OWL is addressed in the Section OWL Compatibility. The semantics of the interaction between RIF and OWL 2 DL is close in spirit to [SWRL].
RIF provides a mechanism for referring to (importing) RDF graphs and a means for specifying the profile of this import, which corresponds to the intended entailment regime. The Section Importing RDF and OWL in RIF specifies how such import statements are used for representing RIFRDF and RIFOWL combinations.
The Appendix: Embeddings (Informative) describes how reasoning with combinations of RIF rules with RDF and OWL 2 RL (a subset of OWL 2 DL) can be reduced to reasoning with RIF documents. This reduction can be seen as an implementation hint for interchange partners who do not have RDF/OWLaware rule systems, but want to process RIF rules that import RDF graphs and OWL ontologies. In terms of the aforementioned scenario: if the interchange partner B does not have an RDF/OWLaware rule system, but B can process RIF rules, then the appendix explains how the rule system of B could be used for processing RIFRDF/OWL combinations.
Throughout this document the following conventions are used when writing RIF and RDF statements in examples and definitions.
Where RDF/OWL has four kinds of constants: URI references (i.e., IRIs), plain literals without language tags, plain literals with language tags and typed literals (i.e., Unicode sequences with datatype IRIs) [RDFConcepts], RIF has one kind of constants: Unicode sequences with symbol space IRIs [RIFDTB].
Symbol spaces can be seen as groups of constants. Every datatype is a symbol space, but there are symbol spaces that are not datatypes. For example, the symbol space rif:iri groups all IRIs. The correspondence between constant symbols in RDF graphs and RIF documents is explained in Table 1.
RDF Symbol  Example  RIF Symbol  Example 

IRI  <http://www.w3.org/2007/rif>  Constant in the rif:iri symbol space  "http://www.w3.org/2007/rif"^^rif:iri 
Plain literal without language tag  "literal string"  Constant in the xs:string symbol space  "literal string"^^xs:string 
Plain literal with language tag  "literal string"@en  Constant in the rdf:PlainLiteral symbol space  "literal string@en"^^rdf:PlainLiteral 
Typed literal  "1"^^xs:integer  Constant with symbol space  "1"^^xs:integer 
The shortcut syntax for IRIs and strings [RIFDTB], used throughout this document, corresponds to the syntax for IRIs and plain literals in Turtle [Turtle], a commonly used syntax for RDF.
RIF does not have a notion corresponding exactly to RDF blank nodes. RIF local symbols, written _symbolname, have some commonality with blank nodes; like the blank node label, the name of a local symbol is not exposed outside of the document. However, in contrast to blank nodes, which are essentially existentially quantified variables, RIF local symbols are constant symbols. In many applications and deployment scenarios, this difference may be inconsequential. However the results will differ when such symbols are used in a nonassertional context, such as in a query pattern or rule body.
Finally, variables in the bodies of RIF rules or in query patterns may be existentially quantified, and are thus similar to blank nodes; however, RIF BLD does not allow existentially quantified variables to occur in rule heads.
This section specifies how a RIF document interacts with a set of RDF graphs in a RIFRDF combination. In other words, how rules can "access" data in the RDF graphs.
There is a correspondence between statements in RDF graphs and certain kinds of formulas in RIF. Namely, there is a correspondence between RDF triples of the form s p o and RIF frame formulas of the form s'[p' > o'], where s', p', and o' are RIF symbols corresponding to the RDF symbols s, p, and o, respectively. This means that whenever a triple s p o is satisfied, the corresponding RIF frame formula s'[p' > o'] is satisfied, and vice versa.
Consider, for example, a combination of an RDF graph that contains the triples
ex:john ex:brotherOf ex:jack . ex:jack ex:parentOf ex:mary .
saying that ex:john is a brother of ex:jack and ex:jack is a parent of ex:mary, and a RIF document that contains the rule
Forall ?x ?y ?z (?x[ex:uncleOf > ?z] : And(?x[ex:brotherOf > ?y] ?y[ex:parentOf > ?z]))
which says that whenever some x is a brother of some y and y is a parent of some z, then x is an uncle of z. From this combination the RIF frame formula :john[:uncleOf > :mary], as well as the RDF triple :john :uncleOf :mary, are consequences of this combination.
Note that blank nodes cannot be referenced directly from RIF rules, since blank nodes are local to a specific RDF graph. Variables in RIF rules do, however, range over objects denoted by blank nodes. So, it is possible to "access" an object denoted by a blank node from a RIF rule using a variable in a rule.
The following example illustrates the interaction between RDF and RIF in the face of blank nodes.
Consider a combination of an RDF graph that contains the triple
_:x ex:hasName "John" .
saying that there is something, denoted here by a blank node, which has the name "John", and a RIF document that contains the rules
Forall ?x ?y ( ?x[rdf:type > ex:named] : ?x[ex:hasName > ?y] ) Forall ?x ?y ( <http://a>[<http://p> > ?y] : ?x[ex:hasName > ?y] )
which says that whenever there is some x that has some name y, then x is of type ex:named and http://a has a property http://p with value y.
From this combination the following RIF condition formulas can be derived:
Exists ?z (?z[rdf:type > ex:named]) <http://a>[<http://p> > "John"]
as can the following RDF triples:
_:y rdf:type ex:named . <http://a> <http://p> "John" .
However, there is no RIF constant symbol t such that t[rdf:type > ex:named] can be derived, because there is no constant that represents the named individual.
Note that, even when considering Simple entailment, not every combination is satisfiable. In fact, not every RIF document has a model. For example, the RIF BLD document consisting of the fact
"a"="b"
does not have a model, since the symbols "a" and "b" are mapped to the (distinct) character strings "a" and "b", respectively, in every semantic structure.
One consequence of the difference of the alphabets of RDF and RIF is that IRIs of the form http://iri and typed literals of the form "http://iri"^^rif:iri that occur in an RDF graph are treated the same in RIFRDF combinations, even if the RIF document is empty. However, documents importing RDF graphs containing typed literals of the form "http://iri"^^rif:iri must be rejected.
Plain literals without language tags of the form "mystring" and typed literals of the form "mystring"^^xs:string also correspond. For example, consider the combination of an empty document and an RDF graph that contains the triple
<http://a> <http://p> "abc" .
This combination entails, among other things, the following frame formula:
<http://a>[<http://p> > "abc"^^xs:string]
as well as the following triple:
<http://a> <http://p> "abc"^^xs:string .
These entailments are sanctioned by the semantics of plain literals and xs:strings.
Lists in RDF (also called collections) have a natural correspondence to RIF lists. For example, the RDF list _:l1 rdf:first ex:b . _:l1 rdf:rest rdf:nil . corresponds to the RIF list List(ex:b). And so, the combination of the empty RIF document with the RDF graph
ex:a ex:p _:l1 . _:l1 rdf:first ex:b . _:l1 rdf:rest rdf:nil .
entails the formula
ex:a[ex:p > List(ex:b)].
Likewise, the combination of the empty RDF graph with the RIF fact
ex:p(List(ex:a))
entails the triples
_:l1 rdf:first ex:a . _:l1 rdf:rest rdf:nil .
as well as the formula
Exists ?x (And(ex:p(?x) ?x[rdf:first > ex:a] ?x[rdf:rest > rdf:nil])).
Editor's Note: This last example shows there is a 1to1 correspondence between RIF and RDF lists. Note that 1to1 correspondence (condition 10 in commonRIFRDFinterpetations) is at risk. The previous example shows that RIF lists extend RDF lists (condition 9 in commonRIFRDFinterpetations); this feature is not at risk.
The remainder of this section formally defines combinations of RIF rules with RDF graphs and the semantics of such combinations. A combination consists of a RIF document and a set of RDF graphs. The semantics of combinations is defined in terms of combined models, which are pairs of RIF and RDF interpretations. The interaction between the two interpretations is defined through a number of conditions. Entailment is defined as model inclusion, as usual.
This section first reviews the definitions of RDF vocabularies and RDF graphs, after which RIFRDF combinations are formally defined. The section concludes with a review of definitions related to datatypes and typed literals.
An RDF vocabulary V consists of the following sets of names:
In addition, there is an infinite set of blank nodes, which is disjoint from the sets of names. See RDF Concepts and Abstract Syntax [RDFConcepts] for precise definitions of these concepts.
Definition. Given an RDF vocabulary V, a generalized RDF triple of V is a statement of the form s p o, where s, p and o are names in V or blank nodes. ☐
Definition. Given an RDF vocabulary V, a generalized RDF graph is a set of generalized RDF triples of V. ☐
(See the End note on generalized RDF graphs)
A RIFRDF combination consists of a RIF document and zero or more RDF graphs. Formally:
Definition. A RIFRDF combination is a pair < R,S>, where R is a RIF document and S is a set of generalized RDF graphs of a vocabulary V. ☐
When clear from the context, RIFRDF combinations are referred to simply as combinations.
Even though RDF allows the use of arbitrary datatype IRIs in typed literals, not all such datatype IRIs are recognized in the semantics. In fact, Simple entailment does not recognize any datatype and RDF and RDFS entailment recognize only the datatype rdf:XMLLiteral. To facilitate discussing datatypes, and specifically datatypes supported in specific contexts (required for Dentailment),RIFDentailment), the notion of datatype maps [RDFSemantics] is used.
A datatype map is a partial mapping from IRIs to datatypes.
RDFS, specifically Dentailment,RIFDentailment, allows the use of arbitrary datatype maps, as long as rdf:XMLLiteral is in the domain of the map. RIF BLD requires a number of additional datatypes to be included; these are the RIFrequired datatypes [RIFDTB].
When checking consistency of a combination < R,S> or entailment of a graph S or RIF formula φ by a combination < R,S>, the set of considered datatypes is the union of the set of RIFrequired datatypes and the sets of datatypes referenced in R, the documents imported into R, and φ (when considering entailment of φ).
Definition. Let DTS be a set of datatypes. A datatype map D is conforming with DTS if it satisfies the following conditions:
Note that it follows from the definition that every datatype used in the RIF document in the combination or the entailed RIF formula (when considering entailment questions) is included in any datatype map conforming to the set of considered datatypes. There may be datatypes used in an RDF graph in the combination that are not included in such a datatype map.
Definition. Given a datatype map D, a typed literal (s, d) is a welltyped literal if
The semantics of RIFRDF combinations is defined through a combination of the RIF and RDF model theories, using a notion of common models. These models are then used to define satisfiability and entailment in the usual way. Combined entailment extends both entailment in RIF and entailment in RDF.
The RDF Semantics document [RDFSemantics] defines four normative kinds of interpretations, as well as corresponding notions of satisfiability and entailment:
Those four types of interpretations are reflected in the definitions of satisfaction and entailment in this section.
This section defines the notion of commonRIFRDFinterpretation, which is an interpretation of a RIFRDF combination. This commonRIFRDFinterpretation is the basis for the definitions of satisfaction and entailment in the following sections.
The correspondence between RIF semantic structures (interpretations) and RDF interpretations is defined through a number of conditions that ensure the correspondence in the interpretation of names (i.e., IRIs and literals) and formulas, i.e., the correspondence between RDF triples of the form s p o and RIF frames of the form s'[p' > o'], where s', p', and o' are RIF symbols corresponding to the RDF symbols s, p, and o, respectively (cf. the Section Symbols in RIF Versus RDF/OWL).
The notions of RDF interpretation and RIF semantic structure (interpretation) are briefly reviewed below.
As defined in [RDFSemantics], a Simple interpretation of a vocabulary V is a tuple I=< IR, IP, IEXT, IS, IL, LV >, where
RDF, RDFS, and Dinterpretations are Simple interpretations that satisfy certain conditions:
As defined in [RIFBLD], a semantic structure I is a tuple of the form <TV, DTS, D, D_{ind}, D_{func}, I_{C}, I_{V}, I_{F}, I_{NF}, I_{list}, I_{tail}, I_{frame}, I_{sub}, I_{isa}, I_{=}, I_{external}, I_{truth}>. The specification of RIFRDF compatibility is only concerned with DTS, D, I_{C}, I_{V}, I_{list}, I_{tail}, I_{frame}, I_{sub}, I_{isa}, and I_{truth}. The other mappings that are parts of a semantic structure are not used in the definition of combinations.
Recall that Const is the set of constant symbols and Var is the set of variable symbols in RIF.
For the purpose of the interpretation of imported documents, RIF BLD defines the notion of semantic multistructures, which are nonempty sets of semantic structures of the form {J,I; I^{i1}, I^{i2}, ...} that differ only in interpretation of local constants. The structure I in the above is used to interpret document formulas, and will be used to specify RIF combinations.
Syntactically speaking, an RDF list is a set of triples of the form
i1 rdf:first d1 . i1 rdf:rest i2 . ... in rdf:first dn . in rdf:rest rdf:nil .
Here, i1 ... in provide the structure of the linked list and d1 ... dn are the items. The above list would be written in RIF syntax as List(d1 ... dn).
Given an RDF interpretation I=< IR, IP, IEXT, IS, IL, LV >, we say that an element l1 ∈ IR refers to an RDF list (y1,...,yn) if l1=IS(rdf:nil), in case n=0; otherwise, ∃ l2, ..., ln such that <l1,y1> ∈ IEXT(IS(rdf:first)), <l1,l2> ∈ IEXT(IS(rdf:rest)), ..., <ln,yn> ∈ IEXT(IS(rdf:first)), and <ln,IS(rdf:nil)> ∈ IEXT(IS(rdf:rest)).
Note that, if n > 0, there may be several lists referred to by l1, since there is no restriction, in general, on the rdf:first elements and the rdf:rest successors.
Definition. A commonRIFRDFinterpretation is a pair (Î, I), where Î is a semantic multistructure of the form {J,I; I^{i1}, I^{i2}, ...}, and I is an RDFa Simple interpretation of a vocabulary V, such that the following conditions hold:
Condition 1 ensures that the combination of resources and properties corresponds exactly to the RIF domain; note that if I is an RDF, RDFS, or Dinterpretation, IP is a subset of IR, and thus IR=D_{ind}. Condition 2 ensures that the set of RDF properties at least includes all elements that are used as properties in frames in the RIF domain. Condition 3 ensures that all concrete values in D_{ind} are included in LV (by definition, the value spaces of all considered datatypes are included in D_{ind}). Condition 4 ensures that RDF triples are interpreted in the same way as frame formulas. Condition 5 ensures that IRIs are interpreted in the same way. Condition 6 ensures that typed literals are interpreted in the same way. Note that no correspondences are defined for the mapping of names in RDF that are not symbols of RIF, e.g., illtyped literals and RDF URI references that are not absolute IRIs. Condition 7 ensures that typing in RDF and typing in RIF correspond, i.e., a rdf:type b is true iff a # b is true. Condition 8 ensures that whenever a RIF subclass statement holds, the corresponding RDF subclass statement holds as well, i.e., a rdfs:subClassOf b is true if a ## b is true. Finally, condition 9 requires the existence of an RIF list for every RDF list and condition 10 in addition requires the existence of an RDF list for every RIF list.
Feature At Risk #1: 1to1 lists
Note: This feature is "at risk" and may be altered or removed from this specification based on feedback. If you have concerns about this or information which may be useful in our eventual decision, please send feedback totell us at publicrifcomments@w3.org. See the full list of features "at risk" in RIF.
Condition 10 (which implies condition 9), which ensures a onetoone correspondence between RDF and RIF lists, is AT RISK and may be removed based on implementation experience.
The notion of satisfiability refers to the conditions under which a commonRIFRDFinterpretation (Î, I) is a model of a combination < R, S>. The notion of satisfiability is defined for all four entailment regimes of RDF (i.e., Simple, RDF, RDFS, and D). The definitions are all analogous. Intuitively, a commonRIFRDFinterpretation (Î, I) satisfies a combination < R, S> if Î is a model of R and I satisfies S. Formally:
Definition. A commonRIFRDFinterpretation (Î, I) satisfies a RIFRDF combination C=< R, S > if Î is a model of R and I satisfies every RDF graph S in S; in this case (Î, I) is called a SimplemodelRIFSimplemodel, or model, of C, and C is satisfiable. (Î, I) satisfies a generalized RDF graph S if I satisfies S. (Î, I) satisfies a condition formula φ if TVal_{Î}(φ)=t. ☐
Rdf, RDFS, and DsatisfiabilityRIFDsatisfiability are defined through additional restrictions on I:
Definition. A model (Î, I) of a combination C is an RDFmodelRIFRDFmodel of C if I is an RDFinterpretation; in this case C is RDFsatisfiableRIFRDFsatisfiable.
A model (Î, I) of a combination C is an RDFSmodelRIFRDFSmodel of C if I is an RDFSinterpretation; in this case C is RDFSsatisfiableRIFRDFSsatisfiable.
Let (Î, I) be a model of a combination C and let D be a datatype map conforming with the set DTS of datatypes in I. (Î, I) is a DmodelRIFDmodel of C if I is a Dinterpretation; in this case C is DsatisfiableRIFDsatisfiable. ☐
Using the notions of models defined above, entailment is defined in the usual way, i.e., through inclusion of sets of models.
Definition. Let C be a RIFRDF combination, let S be a generalized RDF graph, let φ be a condition formula, and let D be a datatype map conforming with the set of considered datatypes. C DentailsRIFDentails S if every DmodelRIFDmodel of C satisfies S. Likewise, C DentailsRIFDentails φ if every DmodelRIFDmodel of C satisfies φ. ☐
The other notions of entailment are defined analogously:
Definition. A combination C SimpleentailsRIFSimpleentails S (resp., φ) if every Simple model of C satisfies S (resp., φ).
A combination C RDFentailsRIFRDFentails S (resp., φ) if every RDFmodelRIFRDFmodel of C satisfies S (resp., φ).
A combination C RDFSentailsRIFRDFSentails S (resp., φ) if every RDFSmodelRIFRDFSmodel of C satisfies S (resp., φ). ☐
Note that simple entailment in combination with an empty ruleset is not the same as simple entailment in RDF, since certain entailments involving datatypes are enforced by the RIF semantics in combinations, cf. the example involving strings and plain literals above.
This section specifies how a RIF document interacts with a set of OWL ontologies in a RIFOWL combination. The semantics of combinations is defined for OWL 2 [OWL2Syntax]. Since OWL 2 is an extension of OWL 1 [OWLReference], the specification in this section applies also to combinations of RIF documents with OWL 1 ontologies.
OWL 2 specifies two different variants of the language, each having its own semantics, namely OWL 2 DL [OWL2Syntax] (called simply OWL 2 in the specification) and OWL 2 Full [OWL2RDFBasedSemantics]. OWL 1 Lite and OWL 1 DL [OWLReference], which are sublanguages of OWL 1, can be seen as syntactical subsets of OWL 2 DL. The key difference between OWL 2 DL and OWL 2 Full is that the semantics of OWL 2 DL [OWL2Semantics] is based on standard Description Logic semantics, whereas the semantics of OWL 2 Full is an extension of the RDFS semantics.
The syntax of OWL 2 is defined in terms of a structural specification, and there is a mapping to an RDF representation for interchange. The RDF representation of OWL 2 DL does not extend the RDF syntax, but rather restricts it: every OWL 2 DL ontology in RDF form is an RDF graph, but not every RDF graph is an OWL 2 DL ontology. OWL 2 Full and RDF have the same syntax: every RDF graph is an OWL 2 Full ontology and vice versa. This syntactical difference is reflected in the definition of RIFOWL compatibility: combinations of RIF with OWL 2 DL are based on the OWL 2 structural model, whereas combinations with OWL 2 Full are based on the RDF syntax.
Since the OWL 2 Full syntax is the same as the RDF syntax and
the OWL 2 Full semantics is an extension of the RDF semantics,
the definition of RIFOWL 2 Full compatibility is an extension
of RIFRDF compatibility. However, defining RIFOWL DL
compatibility in the same way would entail losing certain
semantic properties of OWL 2 DL. One of the main reasons for
this is the difference in the way classes and properties are
interpreted in OWL 2 Full and OWL 2 DL. In the Full variant,
classes and properties are interpreted as objects in the domain
of interpretation, which are then associated with subsets of,
respectively binary relations over the domain of
interpretation, using the rdf:type property and the
extension function IEXT, as in RDF. In the DL variant, classes
and properties are directly interpreted as subsets of,
respectively binary relations over the domain. This is a key
property of the firstorder logic nature of Description Logic
semantics and enables the use of Description Logic reasoning
techniques for processing OWL 2 DL descriptions. Defining
RIFOWL DL compatibility as an extension of RIFRDF
compatibility would define a correspondence between OWL 2 DL
statements and RIF frame formulas. Since RIF frame formulas
are interpreted using an extension function, as in RDF,
defining the correspondence between them and OWL 2 DL
statements would change the semantics of OWL statements, even
if the RIF document were empty.
A RIFOWL combination that is faithful to the firstorder nature of the OWL 2 DL semantics requires interpreting classes and properties as sets and binary relations, respectively, suggesting that a correspondence could be defined with unary and binary predicates. It is, however, also desirable that there be uniform syntax for the RIF component of both RIFOWL 2 DL and RIFRDF/OWL 2 Full combinations, because one may not know at the time of constructing the rules which type of inference will be used. Consider, for example, an RDF graph S consisting of the following statements
_:x rdf:type owl:Ontology . a rdf:type C .
and a RIF document with the rule
Forall ?x (?x[rdf:type > D] : ?x[rdf:type > C])
The combination of the two, according to the specification of RDF Compatibility, allows deriving the triple
a rdf:type D .
Now, the RDF graph S is also an OWL 2 DL ontology. Therefore, one would expect the triple to be implied according to the semantics of RIFOWL DL combinations as well.
To ensure that the RIFOWL DL combination is faithful to the OWL 2 DL semantics and to enable using the same, or similar, RIF rules in combinations with both OWL 2 DL and RDF/OWL 2 Full, the interpretation of frame formulas s[p > o] in RIFOWL DL combinations is slightly different from their interpretation in RIF and syntactical restrictions are imposed on the use of variables and function terms in frame formulas.
The remainder of this section formally defines combinations of
RIF rules with OWL 2 DL and OWL 2 Full ontologies and the
semantics of such combinations. A combination consists of a RIF
document and a set of OWL ontologies. The semantics of
combinations is defined in terms of combined models, which are
pairs of RIF semantic multistructures and OWL 2 DL, respectively OWL 2 Full
interpretations. The interaction between the structures and interpretations is
defined through a number of conditions. Entailment is defined
as model inclusion, as usual.
Since RDF graphs and OWL 2 Full ontologies cannot be distinguished, the syntax of RIFOWL 2 Full combinations is the same as the syntax of RIFRDF combinations.
The syntax of OWL ontologies in RIFOWL DL combinations is given by the structural specification of OWL 2 and the restrictions on OWL 2 DL ontologies OWL2Syntax. Certain restrictions are imposed on the syntax of the RIF rules in combinations with OWL 2 DL. Specifically, the only terms allowed in class and property positions in frame formulas are constant symbols. A DLframe formula is a frame formula a[b_{1} > c_{1} ... b_{n} > c_{n}] such that n≥1 and for every b_{i}, with 1≤i≤n, it holds that b_{i} is a constant symbol and if b_{i} = rdf:type, then c_{i} is a constant symbol.
Definition. A condition formula φ is a DLcondition if every frame formula in φ is a DLframe formula.
A RIFBLD document formula R is a RIFBLD DLdocument formula if every frame formula in R is a DLframe formula.
A RIFOWL DLcombination is a pair < R,O>, where R is a RIFBLD DLdocument formula and O is a set of OWL 2 DL ontologies of a vocabulary V over an OWL 2 datatype map D. ☐
When clear from the context, RIFOWL DLcombinations are referred to simply as combinations.
In the literature, several restrictions on the use of variables in combinations of rules and Description Logics have been identified [Motik05, Rosati06] for the purpose of decidable reasoning. This section specifies such safeness restrictions for RIFOWL2DL combinations.
Given a set of OWL 2 DL ontologies O, a variable
?x in a RIF rule Q H : B is
DLsafe if it occurs in an atomic formula in B
that is not of the form s[Ps[P > o]o] or s[rdf:types[rdf:type >
A]A], where s, P, o, and A are terms (possibly including ?x) and P or A occurs in one of the
ontologies in O. A disjunctionfree RIF rule Q (H : B) is
DLsafe, given O, if every variable that occurs
in H : B is DLsafe. A disjunctionfree RIF rule Q (H : B)
is weakly DLsafe, given O, if every variable
that occurs in H is DLsafe.
Definition. A RIFOWL DLcombination <R,O> is DLsafe if every rule in R is DLsafe, given O. A RIFOWL DLcombination <R,O> is weakly DLsafe if every rule in R is weakly DLsafe, given O. ☐
Feature At Risk #2:#4: Safeness
Note: This feature is "at risk" and may be altered or removed from this specification based on feedback. If you have concerns about this or information which may be useful in our eventual decision, please send feedback totell us at publicrifcomments@w3.org. See the full list of features "at risk" in RIF.
The above definition of DLsafeness is intended to identify a fragment of RIFOWL DL combinations for which reasoning is decidable. This definition should be considered AT RISK and may become stricter based on implementation experience.
Compared with RDF and the RIF, OWL 2 uses a slightly extended notion of datatype.
In the remainder of this section, a datatype d contains, in addition to the lexical space, value space, and lexicaltovalue mapping, a facet space, which is a set of pairs of the form (F, v), where F is an IRI and v is a data value, and a facettovalue mapping, which is a mapping from facets to subsets of the value space of d.
An OWL 2 datatype map D is a datatype map that maps the IRIs of the datatypes specified in Section 4 of [OWL2Syntax] to the corresponding datatypes such that the domain of D does not include rdfs:Literal.
We note here that the definitions of datatype and datatype map in the OWL 2 direct semantics specification
[OWL2Semantics] are somewhat different.
There, a datatype is some entity with some associated IRIs, and
the datatype map assigns lexical value, and facet spaces, as
well as lexicaltovalue and facettovalue mappings. The
definitions of datatype and datatype map we use are isomorphic,
and, indeed, the same as in the OWL 2 RDFbased semantics
specification
[OWL2RDFBasedSemantics].
The latter does not preclude the use of rdfs:Literal
in datatype maps. Note that we do not restrict the use of
rdfs:Literal in OWL 2 ontologies or RDF graphs.
The semantics of RIFOWL 2 Full combinations is a straightforward extension of the Semantics of RIFRDF Combinations.
The semantics of RIFOWL2 DL combinations cannot straightforwardly extend the semantics of RIFRDF combinations, because OWL 2 DL does not extend the RDF semantics. In order to keep the syntax of the rules uniform between RIFOWL 2 Full and RIFOWL DL combinations, the semantics of RIF frame formulas is slightly altered in RIFOWL DL combinations.
Given an OWL 2 datatype map D and a vocabulary V that includes the domain of D and the OWL 2 Full vocabulary and facet names, a Dinterpretation I is an OWL 2 RDFBased Interpretation of V with respect to D if it satisfies the semantic conditions in Section 5 of [OWL2RDFBasedSemantics].
The semantics of RIFOWL 2 Full combinations is a straightforward extension of the semantics of RIFRDF combinations. It is based on the same notion of common interpretations, but defines additional notions of satisfiability and entailment.
Definition. Let (Î, I) be a
commonRIFRDFinterpretation
that is a model of a
RIFRDF combination C=< R,
S > and let D be an OWL 2 datatype map conforming
with the set of datatypes in I. (Î, I) is
an OWLRIFOWL Fullmodel of C
if I is an
OWL 2 RDFBased Interpretation with respect to D; in this case C is
OWLRIFOWL Fullsatisfiable
with respect to D.
Let C be a RIFRDF combination, let S be a
generalized RDF graph, let φ
be a condition formula, and let D be
an OWL 2 datatype map D
conforming with the set of
considered datatypes. C OWLRIFOWL Fullentails
S with respect to D if every
OWLRIFOWL Fullmodel of C
satisfies S. Likewise, C
OWLRIFOWL Fullentails φ with respect to D if every
OWLRIFOWL Fullmodel of C
satisfies φ. ☐
The semantics of RIFOWL DLcombinations is similar in spirit to the semantics of RIFRDF combinations. Analogous to commonRIFRDFinterpretations, there is the notion of commonRIFOWL DLinterpretations, which are pairs of RIF and OWL 2 DL interpretations, and which define a number of conditions that relate these interpretations to each other.
The modification of the semantics of RIF frame formulas is achieved by modifying the mapping function for frame formulas (I_{frame}), and leaving the RIF BLD semantics [RIFBLD] otherwise unchanged.
Namely, frame formulas of the form s[rdf:type > o] are interpreted as membership of s in the set denoted by o and frame formulas of the form s[p > o], where p is not rdf:type, as membership of the pair (s, o) in the binary relation denoted by p.
Definition. A dlsemantic structure is a
tuple I = <TV, DTS,
D, D_{ind},
D_{func}, I_{C},
I_{V}, I_{F},
I_{frame}, I_{NF},
I_{sub}, I_{isa},
I_{=}, I_{external},
I_{truth}>, where
Dlsemantic multistructures are defined analogous to RIFBLD semantic multistructures [RIFBLD]. Formally, a dlsemantic multistructure Î is a set of dlsemantic structures {J,I; I^{i1}, I^{i2}, ...}, where
All the structures in Î (adorned and nonadorned) are identical in all respects except for the following:
The notation I(φ), for any other formula or symbol φ, and the truth valuation function TVal_{Î} are defined as in BLD semantic structures.
Definition. A dlsemantic multistructure Î is a model of a RIFBLD DLdocument formula R if TVal_{Î}(R)=t. ☐
As defined in [OWL2Semantics], an interpretation for a vocabulary V over a datatype map D is a tuple I=< IR, LV, C, OP, DP, I, DT, LT, FA >, where
The OWL semantics imposes a number of further restrictions on the mapping functions to ensure the interpretation of datatypes, literals, and facets conforms with the given datatype map D and to define the semantics of builtin classes and properties (e.g., owl:Thing). The mappings DT, LT, and FA are essentially given by the datatype map.
Definition. Given a vocabulary V over an OWL 2 datatype map D, a commonRIFOWL DLinterpretation for V over D is a pair (Î, I), where Î is a dlsemantic multistructure of the form {J,I; I^{i1}, I^{i2}, ...}, and I is an interpretation for V over D, such that the following conditions hold.
Condition 2 ensures that the relevant parts of the domains of interpretation are the same. Condition 3 ensures that the interpretation (extension) of an OWL class identified by an IRI u corresponds to the interpretation of frames of the form ?x[rdf:type > <u>]. Conditions 4 and 5 ensure that the interpretation (extension) of an OWL object or data property identified by an IRI u corresponds to the interpretation of frames of the form ?x[<u> > ?y]. Finally, condition 6 ensures that individual identifiers in the OWL ontologies and the RIF documents are interpreted in the same way.
Using the definition of commonRIFOWL DLinterpretation,
satisfaction, models, and entailment are defined in the usual
way:
Definition. A
commonRIFOWL DLinterpretation
(Î, I) for a vocabulary V over an
OWL 2 datatype map D is an OWLRIFOWL DLmodel of a
RIFOWL DLcombination C=<
R, O > if Î is a model
of R and I is a model of every ontology O in
O; in this case C is OWLRIFOWL DLsatisfiable for
V over D. (Î, I) is an
OWLRIFOWL DLmodel of an OWL 2 ontology O if I is a
model of O. (Î, I) is an
OWLRIFOWL DLmodel of a DLcondition formula φ if TVal_{Î}(φ)=t.
Let C be a RIFOWL DLcombination, let O be an OWL 2
ontology, let φ be a DLcondition
formula, and let D be an OWL 2
datatype map conforming with
the set of considered datatypes,
and let V be a vocabulary over D for every ontology in C
and for O. C OWLRIFOWL DLentails O
with respect to D if every commonRIFOWL DLinterpretation
for V over D that is an OWLRIFOWL DLmodel
of C is an OWLRIFOWL DLmodel of O.
Likewise, C OWLRIFOWL DLentails φ with respect to D
if every commonRIFOWL DLinterpretation for V over D
that is an OWLRIFOWL DLmodel of C is an
OWLRIFOWL DLmodel of φ.
☐
Example. In OWL 2 DL, the domains for interpreting
individuals respectively, literals (data values), are disjoint.
The disjointness entails that data values cannot be members of
a class and individuals cannot be members of a datatype.
RIF does not make such distinctions; variable quantification ranges over the entire domain. So, the same variable may be assigned to an abstract individual or a concrete data value. Additionally, RIF constants (e.g., IRIs) denoting individuals can be written in place of a data value, such as the value of a datavalued property or in datatype membership statements; similarly for constants denoting data values. Such statements cannot be satisfied in any commonRIFOWL DLinterpretation. The following example illustrates several such statements.
Consider the datatype xs:string and a RIFOWL DL combination consisting of the set containing only the OWL DL ontology
ex:myiri rdf:type ex:A .
and a RIF document containing the following fact
ex:myiri[rdf:type > xs:string]
This combination is not
OWLRIFOWL DLsatisfiable, because
ex:myiri is an individual identifier and S maps individual
identifiers to elements in O, which is disjoint from the
elements in the datatype xs:string.
Consider a RIFOWL DL combination consisting of the set containing only the OWL 2 DL ontology
ex:hasChild rdf:type owl:ObjectProperty .
and a RIF document containing the following fact
ex:myiri[ex:hasChild > "John"]
This combination is not
OWLRIFOWL DLsatisfiable, because
ex:hasChild is an object property, and values of object
properties may not be concrete data values.
Consider a RIFOWL DL combination consisting of the OWL DL ontology
SubClassof(ex:A ex:B)
and a RIF document containing the following rule
Forall ?x (?x[rdf:type > ex:A])
This combination is not
OWLRIFOWL DLsatisfiable, because the
rule requires every element, including every concrete data
value, to be a member of the class ex:A. However,
since every OWL interpretation requires every member of
ex:A to be an element of the object domain, concrete
data values cannot be members of the object domain. ☐
In the preceding sections, RIFRDF Combinations and RIFOWL combinations were defined in an abstract way, as pairs consisting of a RIF document and a set of RDF graphs/OWL ontologies. In addition, different semantics were specified based on the various RDF and OWL entailment regimes. RIF provides a mechanism for explicitly referring to (importing) RDF graphs from documents and specifying the intended profile (entailment regime) through the use of Import statements.
This section specifies how RIF documents with such import statements must be interpreted.
A RIF document contains a number of Import statements. Unary Import statements are used for importing RIF documents, and the interpretation of these statements is defined in Section 3.5 of [RIFBLD]. This section defines the interpretation of binary Import statements:
Import(<t1> <p1>) ... Import(<tn> <pn>)
Here, ti is an absolute IRI referring to an RDF graph to be imported and pi is an absolute IRI denoting the profile to be used for the import.
The profile determines which notions of model, satisfiability and entailment must be used. For example, if a RIF document R imports an RDF graph S with the profile RDFS, the notions of RDFSmodelRIFRDFSmodel, RDFSsatisfiabilityRIFRDFSsatisfiability, and RDFSentailmentRIFRDFSentailment must be used for the combination <R, {S}>.
Profiles are ordered as specified in Section 5.1.1. If several graphs are imported in a document, and these imports specify different profiles, the highest of these profiles is used. For example, if a RIF document R imports an RDF graph S_{1} with the profile RDF and an RDF graph S_{2} with the profile OWL Full, the notions of OWLRIFOWL Fullmodel, OWLRIFOWL Fullsatisfiability, and OWLRIFOWL Fullentailment must be used with the combination <R, {S_{1}, S_{2}}>.
Finally, if a RIF document R imports an RDF graph S with the profile OWL DL, R must be a RIFBLD DLdocument formula, S must be
the RDF representation of an OWL 2 ontology O, and the notions of OWLRIFOWL DLmodel, OWLRIFOWL DLsatisfiability, and OWLRIFOWL DLentailment must be used with the combination <R, {O}>.
RIF defines specific profiles for the different notions of model, satisfiability and entailment of combinations, as well as one generic profile. The use of a specific profile specifies how a combination should be interpreted. If a specific profile cannot be handled by a receiver, the combination should be rejected. The use of a generic profile implies that a receiver may interpret the combination to the best of its ability.
The use of profiles is not restricted to the profiles specified in this document. Any specific profile that is used with RIF must specify an IRI that identifies it, as well as associated notions of model, satisfiability, and entailment for combinations.
The following table lists the specific profiles defined by RIF, the IRIs of these profiles, and the notions of model, satisfiability, and entailment that must be used with the profile.
Profile  IRI of the Profile  Model  Satisfiability  Entailment 

Simple  http://www.w3.org/2007/rifimportprofile#Simple  
satisfiability  
RDF  http://www.w3.org/2007/rifimportprofile#RDF  
RIFRDFsatisfiability  RIFRDFentailment 
RDFS  http://www.w3.org/2007/rifimportprofile#RDFS  
RIFRDFSsatisfiability  RIFRDFSentailment 
D  http://www.w3.org/2007/rifimportprofile#D  
RIFDsatisfiability  RIFDentailment 
OWL DL  http://www.w3.org/2007/rifimportprofile#OWLDL  


OWL Full  http://www.w3.org/2007/rifimportprofile#OWLFull  


Profiles that are defined for combinations of DLdocument formulas and OWL ontologies in abstract syntax form are called DL profiles. Of the mentioned profiles, the profile OWL DL is a DL profile.
The profiles are ordered as follows, where '<' reads "is lower than":
Simple < RDF < RDFS < D < OWL Full
OWL DL < OWL Full
RIF specifies one generic profile. The use of the generic profile does not imply the use of a specific notion of model, satisfiability, and entailment.
Profile  IRI of the Profile 

Generic  <http://www.w3.org/2007/rifimportprofile#Generic> 
Let R be a RIF document such that
Import(<u1> <p1>) ... Import(<un> <pn>)
are all the twoary import statements in R and the documents imported into R and let Profile be the set of profiles corresponding to the IRIs p1,...,pn.
If pi, 1 ≤ i ≤ n, corresponds to a DL profile and ui refers to an RDF graph that is not the RDF representation of an OWL (2) DL ontology, the document should be rejected.
If ui, 1 ≤ i ≤ n, refers to an RDF graph that uses a typed literal of the form "s"^^rif:iri or "s"^^rdf:PlainLiteral, the document must be rejected.
If Profile contains only specific profiles, then:
If Profile contains a generic profile, then the combination C=<R,{S_{1},....,S_{n}}>, where S_{1},....,S_{n} are the RDF graphs referred to by u_{1},...,u_{n}, may be interpreted according to the highest among the specific profiles in Profile, if there is one.
We define notions of conformance for RIFRDF and RIFOWL combinations. We define these notions both for the RIF Core [RIFCore] and RIF BLD [RIFBLD] dialects.
Conformance is described in terms of semanticspreserving transformations between the native syntax of a compliant processor and the XML syntax of RIF Core and BLD.
We say that an RDF graph S is a standard RDF graph if for every triple s p o in S, s is an IRI or blank node, p is an IRI, and o is an IRI, literal, or blank node. A combination < R, S > is standard if every graph in S is standard.
Each RIF processor has sets Τ, of supported datatypes and symbol spaces that include the symbol spaces listed in [RIFDTB], and Ε, of supported external terms that include the builtins listed in [RIFDTB]. The datatype map of a RIF processor is the smallest datatype map conforming with the set of datatypes in Τ.
Now, let P ∈ {Simple, RDF, RDFS, D, OWL Full} be a specific RDF profile. A RIFRDF combination C=< R, S > is a BLD_{Τ,Ε}P combination if R is a BLD_{Τ,Ε} formula and C is a Core_{Τ,Ε}P combination if R is a Core_{Τ,Ε} formula.
A RIFOWL DLcombination C=< R, O > is a BLD_{Τ,Ε}OWL DL combination if R is a BLD_{Τ,Ε} formula and C is a Core_{Τ,Ε}OWL DL combination if R is a Core_{Τ,Ε} formula.
A RIF processor is a conformant BLD_{Τ,Ε}P consumer, for P ∈ {Simple, RDF, RDFS, D, OWL DL, OWL Full}, iff it implements a semanticspreserving mapping, μ, from the set of standard BLD_{Τ,Ε}P combinations, standard RDF graphs, OWL 2 ontologies, and BLD_{Τ,Ε} formulas to the language L of the processor (μ does not need to be an "onto" mapping) and, in case P ∈ {OWL DL, OWL Full}, its datatype map is an OWL 2 datatype map.
We say that a RIF document R is listsafe if R is safe and it contains no occurrences of rdf:first, rdf:rest, or rdf:nil in rule consequents. An RDF graph S is listsafe if it contains no occurrences of rdf:first or rdf:rest outside of the property positions, it contains no occurrences of rdf:nil outside of triples of the form ... rdf:rest rdf:nil, and there are no two triples s rdf:first o1 . s rdf:first o2 . or s rdf:rest o1 . s rdf:rest o2 . in S, where s, o1, o2 are RDF terms and o1≠o2. A combination < R, S > is listsafe if R is listsafe and the merge of the graphs in S is listsafe.
A RIF processor is a conformant Core_{Τ,Ε}P consumer, for P ∈ {Simple, RDF, RDFS, D, OWL DL, OWL Full}, iff it implements a semanticspreserving mapping, μ, from the set of standard listsafe Core_{Τ,Ε}P combinations, standard RDF graphs, OWL 2 ontologies, and Core_{Τ,Ε} formulas to the language L of the processor (μ does not need to be an "onto" mapping) and, in case P ∈ {OWL DL, OWL Full}, its datatype map is an OWL 2 datatype map.
Formally, this means that for any pair (φ, ψ), where φ is a BLD_{Τ,Ε}P combination and ψ is an RDF graph, OWL 2 ontology, or BLD_{Τ,Ε} formula such that φ =_{P} ψ is defined, φ =_{P} ψ iff μ(φ) =_{L} μ(ψ). Here =_{P} denotes Pentailment and =_{L} denotes the logical entailment in the language L of the RIF processor.
A RIF processor is a conformant BLD_{Τ,Ε}P producer iff it implements a semanticspreserving mapping, ν, from the language L of the processor to the set of all BLD_{Τ,Ε} formulas, RDF graphs, OWL 2 ontologies, and BLD_{Τ,Ε}P combinations (ν does not need to be an "onto" mapping).
A RIF processor is a conformant Core_{Τ,Ε}P producer iff it implements a semanticspreserving mapping, ν, from the language L of the processor to the set of all Core_{Τ,Ε} formulas, RDF graphs, OWL 2 ontologies, and Core_{Τ,Ε}P combinations (ν does not need to be an "onto" mapping).
Formally, this means that for any pair (φ, ψ) of formulas in L such that φ =_{L} ψ is defined, φ =_{L} ψ iff ν(φ) =_{P} ν(ψ). Here =_{P} denotes Pentailment and =_{L} denotes the logical entailment in the language L of the RIF processor.
This document is the product of the Rules Interchange Format (RIF) Working Group (see below), the members of which deserve recognition for their time and commitment to RIF. The editors extend special thanks to: Mike Dean, Michael Kifer, Stella Mitchell, Axel Polleres, and Dave Reynolds, for their thorough reviews and insightful discussions; the working group chairs, Chris Welty and Christian de SainteMarie, for their invaluable technical help and inspirational leadership; and W3C staff contact Sandro Hawke, a constant source of ideas, help, and feedback.
The following members of the joint RIFOWL task force have contributed to the OWL Compatibility section in this document: Mike Dean, Peter F. PatelSchneider, and Ulrike Sattler.
The regular attendees at meetings of the Rule Interchange Format (RIF) Working Group at the time of the publication were:
Adrian Paschke (Freie Universitaet Berlin),
Axel Polleres (DERI),
Changhai Ke (ILOG),
Chris Welty (IBM),
Christian de Sainte Marie (ILOG),
Dave Reynolds (HP),
Gary Hallmark (ORACLE),
Harold Boley (NRC),
Hassan AïtKaci (ILOG),
Jos de Bruijn (FUB),
Leora Morgenstern (IBM),
Michael Kifer (Stony Brook),
Mike Dean (BBN),
Sandro Hawke (W3C/MIT), and
Stella Mitchell (IBM).
RIFRDF combinations can be embedded into RIF documents in a fairly straightforward way, thereby demonstrating how a RIFcompliant translator without native support for RDF can process RIFRDF combinations. RIFOWL combinations cannot be embedded in RIF, in the general case. However, there is a subset of OWL 2 DL, namely the OWL 2 RL profile [OWL2Profiles], for which RIFOWL combinations that can be embedded.
Simple, RDF, and RDFS entailment for RIFRDF combinations are embedded in RIF Core. RIFOWL 2 RL combinations require reasoning with equality, and thus could not be embedded in RIF Core; they are embedded in RIF BLD.
The embeddings are defined using an embedding function tr that maps symbols, triples, and RDF graphs/OWL ontologies to RIF symbols, statements, and documents, respectively.
To embed consistency checking in RDF(S) and OWL, we use a special 0ary predicate symbol rif:error, which is assumed not to be used in the RIF documents in the combination.
Besides the namespace prefixes defined in the Overview, the following namespace prefix is used in this appendix: pred refers to the RIF namespace for builtin predicates http://www.w3.org/2007/rifbuiltinpredicate# [RIFDTB].
To facilitate the definition of the embeddings we define the notion of a merge of RIF formulas.
Definition. Let R={R_{1},...,R_{n}} be a set of document, group, and rule formulas, such that there are no prefix or base directives, or relative IRIs in R and directive_{11}, ..., directive_{nm} are all the import directives occurring in document formulas in R. The merge of R, denoted merge(R), is defined as Document(directive_{11} ... directive_{nm} Group(R*_{1} ... R*_{n})), where R*_{i} is obtained from R_{i} in the following way:
Note that the requirement that no prefix or based directives, or relative IRIs are included in any of the formulas to be merged is not a limitation, since compact IRIs can be rewritten to absolutes IRIs, as can relative IRIs, by exploiting prefix and base directives, and the location of the document.
RIFRDF combinations are embedded by combining the RIF rules with embeddings of the RDF graphs and an axiomatization of Simple, RDF, and RDFS entailment.
The embedding is not defined for combinations that include infinite RDF graphs and for combinations that include RDF graphs with RDF URI references that are not absolute IRIs (see the End note on RDF URI references) or plain literals that are not in the lexical space of the xs:string datatype [XMLSchema2]. Also, the embedding is not defined for RDF lists.
We define a listfree combination as a combination that does not contain any mention of the symbols rdf:first, rdf:rest, or rdf:nil.
In the remainder of this section we first define the embedding of symbols, triples, and graphs, after which we define the axiomatization of Simple, RDF, and RDFS entailment of combinations and, finally, demonstrate faithfulness of the embeddings.
Given a combination C=< R,S>, the function tr maps RDF symbols of a vocabulary V and a set of blank nodes B to RIF symbols, as defined in the following table. It is assumed that the vocabulary V includes all the IRIs and literals used in the RIF documents and condition formulas under consideration.
In the table, the mapping tr' is an injective function that maps typed literals to new constants in the rif:local symbol space, where a new constant is a constant that is not used in the document or its vicinity (i.e., entailed formula or entailing combination). It "generates" a new constant from a typed literal.
RDF Symbol  RIF Symbol  Mapping 

IRI i in V_{U}  Constant with symbol space rif:iri  tr(i) = <i> 
Blank node _:x in B  Variable symbol ?x  tr(_:x) = ?x 
Plain literal without a language tag xxx in V_{PL}  Constant with the datatype xs:string  tr("xxx") = "xxx" 
Plain literal with a language tag "xxx"@lang in V_{PL}  Constant with the datatype rdf:PlainLiteral  tr("xxx"@lang) = "xxx@lang"^^rdf:PlainLiteral 
Welltyped literal "s"^^u in V_{TL}  Constant with the symbol space u  tr("s"^^u) = "s"^^u 
Nonwelltyped literal "s"^^u in V_{TL}  Local constant su' that is not used in C and is obtained from "s"^^u  tr("s"^^u) = tr'("s"^^u) 
This section extends the mapping function tr to triples and defines two embedding functions for RDF graphs. In one embedding (tr_{R}), graphs are embedded as RIF documents and variables (originating from blank nodes) are skolemized, i.e., replaced with new constant symbols. In the other embedding (tr_{Q}), graphs are embedded as condition formulas and variables (originating from blank nodes) are existentially quantified. The following sections show how these embeddings can be used for reasoning with combinations.
For skolemization we assume a function sk that takes as argument a formula φ and returns a formula φ' that is obtained from φ by replacing every variable symbol ?x with <newiri>, where newiri is a new globally unique IRI, i.e., it does not occur in the graph or its vicinity (i.e., entailing combination or entailed graph/formula).
RDF Construct  RIF Construct  Mapping 

Triple s p o .  Frame formula tr(s)[tr(p) > tr(o)]  tr(s p o .) = tr(s)[tr(p) > tr(o)] 
Graph S  Group formula tr_{R}(S)  tr_{R}(S) = sk(Document (Group (tr(t_{1}) ... tr(t_{m})) )), where t_{1}, ..., t_{m} are the triples in S 
Graph S  Condition formula tr_{Q}(S)  tr_{Q}(S) = Exists tr(x_{1}) ... tr(x_{n}) (And(tr(t_{1}) ... tr(t_{m}))), where x_{1}, ..., x_{n} are the blank nodes occurring in S and t_{1}, ..., t_{m} are the triples in S 
The semantics of the RDF vocabulary does not need to be axiomatized for Simple entailment. Nonetheless, the connection between RIF class membership and subclass statements and the RDF type and subclass statements needs axiomatization. We define:
R^{Simple}  =  Document( Group( Forall ?x ?y (?x[rdf:type > ?y] : ?x # ?y) Forall ?x ?y (?x # ?y : ?x[rdf:type > ?y]) Forall ?x ?y (?x[rdfs:subClassOf > ?y] : ?x ## ?y]) )) 
The following theorem shows how checking SimpleentailmentRIFSimpleentailment of combinations can be reduced to checking entailment of RIF conditions by using the embeddings of RDF graphs defined above.
Theorem A listfree RIFRDF combination C=<R, R^{Simple},{S_{1},...,S_{n}}> SimpleentailsRIFSimpleentails a generalized RDF graph T if and only if merge({R, tr_{R}(S_{1}), ..., tr_{R}(S_{n})}) entails tr_{Q}(T);
C SimpleentailsRIFSimpleentails a condition formula φ if and only if merge({R, R^{Simple}, tr_{R}(S_{1}), ..., tr_{R}(S_{n}}) entails φ.
Proof. We prove both directions through contraposition. We first consider condition formulas (the second part of the theorem), after which we consider graphs (the first part of the theorem).
In the proof we abbreviate merge({R, R^{Simple}, tr_{R}(S_{1}), ..., tr_{R}(S_{n})}) with R'.
(=>) Assume R' does not entail φ. This means there is some semantic multistructure Î that is a model of R', but not of φ. Consider the pair (Î, I), where I is the interpretation defined as follows:Clearly, (Î, I) is a commonRIFRDFinterpretation: conditions 16 in the definition are satisfied by construction of I and conditions 7 and 8 are satisfied by condition 4 and by the fact that Î is a model of R^{Simple}.
 IR is D_{ind},
 IP is the set of all k in D_{ind} such that there exist some a, b in D_{ind} and I_{truth}(I_{frame}(a)(k,b))=t,
 LV is the union of the value spaces of all considered datatypes,
 IEXT(k) is the set of all pairs (a, b), with a, b, and k in D_{ind}, such that I_{truth}(I_{frame}(a)(k,b))=t,
 IS(i) is I_{C}(<i>), for every absolute IRI i in V_{U}, and
 IL((s, d)) is I_{C}(tr("s"^^d)), for every typed literal (s, d) in V_{TL}.
Consider a graph S_{i} in {S_{1},...,S_{n}}. Let x_{1},..., x_{m} be the blank nodes in S_{i} and let u_{1},..., u_{m} be the new IRIs that were obtained from the variables ?x_{1},..., ?x_{m} through the skolemization in tr_{R}(S_{i}), i.e., u_{i}=sk(?x_{i}). Now, let A be a mapping from blank nodes to elements in D_{ind} such that A(x_{j})=I_{C}(u_{j}) for every blank node x_{j} in S_{i}. From the fact that I is a model of tr_{R}(S_{i}) and by construction of I it follows that [I+A] satisfies S_{i} (see Section 1.5 of [RDFSemantics])), and so I satisfies S_{i}.
We have that Î is a model of R, by assumption. So, (Î, I) satisfies C. Again, by assumption, I is not a model of φ. Therefore, C does not entail φ.
Assume now that R' does not entail tr_{Q}(T), which means there is a semantic multistructure Î that is a model of R', but not of tr_{Q}(T). The commonRIFRDFinterpretation (Î, I) is obtained in the same way as above, and so it satisfies C.
We proceed by contradiction. Assume I satisfies T. This means there is some mapping A from the blank nodes x_{1},...,x_{m} in T to objects in D_{ind} such that [I+A] satisfies T. Consider now the semantic multistructure Î*, which is the same as Î, with the exception of the mapping I*_{V} on the variables ?x_{1},...,?x_{m}, which is defined as follows: Î*_{V}(?x_{j})=A(x_{j}) for each blank node x_{j} in S. By construction of I and since [I+A] satisfies T we can conclude that I* is a model of And(tr(t_{1})... tr(t_{m})), and so I is a model of tr_{Q}(T), violating the assumption that it is not. Therefore, (Î, I) does not satisfy T and C does not entail T.
(<=) Assume C does not Simpleentail φ. This means there is some commonRIFRDFinterpretation (Î, I) that satisfies C such that I is not a model of φ.
Consider the semantic multistructure Î', which is like Î, except for the mapping I'_{C} on the new IRIs that were introduced by the skolemization mapping sk(). The mapping of these new IRIs is defined as follows:
For each graph S_{i} in {S_{1},...,S_{n}}, let x_{1},..., x_{m} be the blank nodes in S_{i} and let u_{1},..., u_{m} be the new IRIs that were obtained from the variables ?x_{1},..., ?x_{m} through the skolemization in tr_{R}(S_{i}), i.e., u_{j}=sk(?x_{j}). Now, since I satisfies S_{i}, there must be a mapping A from blank nodes to elements in D_{ind} such that [I+A] satisfies S_{i}. We define I'_{C}(u_{j})=A(x_{j}) for every blank node x_{j} in S_{i}.
By assumption, Î' is a model of R (recall that Î' differs from Î only on the new IRIs, which are not in R). Clearly, I' is also a model of R^{Simple}, by conditions 7, 8, and 4 in the definition of commonRIFRDFinterpretation. From the fact that I satisfies S_{i} and by construction of I' it follows that I' is a model of tr_{R}(S_{i}). So, I' is a model of R'. Since I is not a model of φ and φ does not contain any of the new IRIs, I' is not the model of φ. Therefore, R' does not entail φ.
Assume now that C does not entail T, which means there is a commonRIFRDFinterpretation (Î, I) that satisfies C, but I does not satisfy T. We obtain I' from I in the same way as above, and so it satisfies R'. It can be shown analogous to the (=>) direction that if I' is a model of tr_{Q}(T), then there is a blank node mapping A such that [I+A] satisfies T, and thus I satisfies T, violating the assumption that it does not. Therefore, I' is not a model of tr_{Q}(T) and thus R' does not entail tr_{Q}(T). ☐
Theorem A listfree RIFRDF combination <R,{S_{1},...,S_{n}}> is satisfiable iff there is a semantic multistructure Î that is a model of merge({R, R^{Simple}, tr_{R}(S_{1}), ..., tr_{R}(S_{n})}).
Proof. The theorem follows immediately from the previous theorem and the observation that a combination (respectively, RIF document) is satisfiable (respectively, has a model) if and only if it does not entail the condition formula "a"="b". ☐
We axiomatize the semantics of the RDF vocabulary using the following RIF rules.
To finitely embed RDF entailment, we need to consider a subset of the RDF axiomatic triples. Given a combination C, the context of C includes C and its vicinity (i.e., all graphs/formulas considered for entailment checking). The set of RDF finiteaxiomatic triples is the smallest set such that:
We assume that the unary predicate symbols ex:wellxml and ex:illxml are not used in the context of the the given combination.
R^{RDF}  =  merge ((R^{Simple}) union ((tr(s p o .)) for every RDF finiteaxiomatic triple s p o .) union Forall ?x (?x[rdf:type > rdf:XMLLiteral] : ex:wellxml(?x)), Forall ?x (rif:error : And(?x[rdf:type > rdf:XMLLiteral] ex:illxml(?x))), Forall ?x (rif:error : And(ex:illxml(?x) Exists ?y (pred:isLiteralOfType(?x ?y)))) 
Here, inconsistencies may occur if nonwelltyped XML literals, axiomatized using the ex:illxml predicate, are in the class extension of rdf:XMLLiteral. If this situation occurs, rif:error is derived, which signifies an inconsistency in the combination.
Theorem An RDFsatisfiableRIFRDFsatisfiable listfree RIFRDF combination C=<R,{S_{1},...,S_{n}}> RDFentailsRIFRDFentails a generalized RDF graph T iff merge({R^{RDF}, R, tr_{R}(S_{1}), ..., tr_{R}(S_{n})}) entails tr_{Q}(T).
C rdfentails a condition formula φ iff merge({R^{RDF}, R, tr_{R}(S_{1}), ..., tr_{R}(S_{n})}) entails φ.
Proof. In the proof we abbreviate merge({R^{RDF}, R, tr_{R}(S_{1}), ..., tr_{R}(S_{n})}) with R'.
The proof is obtained from the proof of correspondence for Simple entailment in the previous section with the following modifications: (*) in the (=>) direction we additionally need to ensure that I does not satisfy rif:error, extend I to ensure it satisfies the RDF axiomatic triples and show that I is an RDFinterpretation, and (**) in the (<=) direction we need to slightly extend the definition of I' to account for ex:wellxml and ex:illxml, and show that I' is a model of R^{RDF}.
(*) We assume that, for every nonwelltyped literal of the form (s, rdf:XMLLiteral) in V_{TL}, I_{C}(tr("s"^^rdf:XMLLiteral)) is not in the value space of any of the considered datatypes and tr("s"^^rdf:XMLLiteral)[rdf:type > rdf:XMLLiteral] is not satisfied in I. Since C isRDFsatisfiable,RIFRDFsatisfiable, one can verify that this does not compromise satisfaction of R'. Finally, we may assume, without loss of generality, that I does not satisfy rif:error. See also the proof of the following theorem.
For any positive integer j such that rdf:_j does not occur in the context of C, I and I are extended such that IS(rdf:_j)=I_{C}(rdf:_j)=I_{C}(rdf:_m) (see the definition of finiteaxiomatic triples above for the definition of m). Clearly, this does not affect satisfaction of R' or nonsatisfaction of φ and tr_{Q}(T).
To show that I is an RDFinterpretation, we need to show that I satisfies the RDF axiomatic triples and the RDF semantic conditions.
Satisfaction of the axiomatic triples follows immediately from the inclusion of tr(t) in R^{RDF} for every RDF finiteaxiomatic triple t, the fact that I is a model of R^{RDF}, and construction of I. Consider the three RDF semantic conditions:
1 x is in IP if and only if <x, I(rdf:Property)> is in IEXT(I(rdf:type)) 2 If "xxx"^^rdf:XMLLiteral is in V and xxx is a welltyped XML literal string, then
(a) IL("xxx"^^rdf:XMLLiteral) is the XML value of xxx;
(b) IL("xxx"^^rdf:XMLLiteral) is in LV;
(c) IEXT(I(rdf:type)) contains <IL("xxx"^^rdf:XMLLiteral), I(rdf:XMLLiteral)>3 If "xxx"^^rdf:XMLLiteral is in V and xxx is an illtyped XML literal string, then
(a) IL("xxx"^^rdf:XMLLiteral) is not in LV;
(b) IEXT(I(rdf:type)) does not contain <IL("xxx"^^rdf:XMLLiteral), I(rdf:XMLLiteral)>.
Satisfaction of condition 1 follows from satisfaction of the first rule in R^{RDF} in I and construction of I; specifically, the second bullet in the definition.
Consider a welltyped XML literal "xxx"^^rdf:XMLLiteral. By the definition of satisfaction in RIF BLD, I_{C}("xxx"^^rdf:XMLLiteral) is the XML value of xxx (condition 2a), and is clearly in LV (condition 2b), by definition of I. Condition 2c is satisfied by satisfaction of the second rule in R^{RDF} in I.
Satisfaction of 3a and 3b follows straightforwardly from our assumptions on I. This establishes the fact that I is an RDFinterpretation.
(**) Recall that, by assumption, ex:wellxml and ex:illxml are not used in R. Therefore, changing satisfaction of atomic formulas involving ex:wellxml and ex:illxml does not affect satisfaction of R. We assume that I'_{C}(ex:wellxml)=k and I'_{C}(ex:illxml)=l are distinct unique elements, i.e., no other constants is mapped to k and l.
We define I'_{F}(k) and I'_{F}(l) as follows: For every typed literal of the form (s, rdf:XMLLiteral) such that I'_{C}(tr(s^^rdf:XMLLiteral))=u, if (s, rdf:XMLLiteral) is welltyped, I_{truth}(I'_{F}(k)(u))=t and I_{truth}(I'_{F}(l)(u))=f, otherwise I_{truth}(I'_{F}(k)(u))=f and I_{truth}(I'_{F}(l)(u))=t; I'_{truth}(I'_{F}(k)(v))=I_{truth}(I'_{F}(l)(v))=f for every other object v in D_{ind}.Consider R^{RDF}. Satisfaction of R^{Simple} was established in the proof in the previous section. Satisfaction of the facts corresponding to the RDF axiomatic triples in I' follows immediately from the definition of commonRIFRDFinterpretation and the fact that I is an RDFinterpretation, and thus satisfies all RDF axiomatic triples.
Satisfaction of the ex:wellxml and ex:illxml facts in R^{RDF} follows immediately from the definition of I'. Finally, satisfaction of the rules in R^{RDF} follow straightforwardly from the RDF semantic conditions 1, 2, and 3. This establishes the fact that I' is a model of R^{RDF}. ☐
Theorem A listfree RIFRDF combination <R,{S_{1},...,S_{n}}> is RDFsatisfiableRIFRDFsatisfiable iff merge({R^{RDF}, R, tr_{R}(S_{1}), ..., tr_{R}(S_{n})}) does not entail rif:error.
Proof. Recall that we assume rif:error does not occur in R. If <R,{S_{1},...,S_{n}}> is notRDFsatisfiable,RIFRDFsatisfiable, then either merge({R, tr_{R}(S_{1}), ..., tr_{R}(S_{n})}) is not consistent, or condition 3a or 3b (see previous proof) is violated. In either case, rif:error is entailed. If rif:error is entailed, either merge({R^{RDF}, R, tr_{R}(S_{1}), ..., tr_{R}(S_{n})}) is inconsistent, which means merge({R, tr_{R}(S_{1}), ..., tr_{R}(S_{n})}) is not consistent and thus <R,{S_{1},...,S_{n}}> is notRDFsatisfiable,RIFRDFsatisfiable, or the body of the second or third rule in R^{RDF} is satisfied in every model, which means either condition 3a or 3b is violated, and so <R,{S_{1},...,S_{n}}> is notRDFsatisfiable.RIFRDFsatisfiable. ☐
We axiomatize the semantics of the RDF(S) vocabulary using the following RIF rules.
Similar to the RDF case, the set of RDFS finiteaxiomatic triples is the smallest set such that:
We assume that the unary predicate symbol ex:welllit is not used in the context of the given combination.
R^{RDFS}  =  merge((R^{RDF}) union ((tr(s p o .) for every RDFS finiteaxiomatic triple s p o .) union Forall ?x (?x[rdf:type > rdfs:Resource] : Exists ?y ?z (?z[?y > ?x])), Forall ?u ?v ?x ?y (?u[rdf:type > ?y] : And(?x[rdfs:domain > ?y] ?u[?x > ?v])), Forall ?u ?v ?x ?y (?v[rdf:type > ?y] : And(?x[rdfs:range > ?y] ?u[?x > ?v])), Forall ?x (?x[rdfs:subPropertyOf > ?x] : ?x[rdf:type > rdf:Property]), Forall ?x ?y ?z (?x[rdfs:subPropertyOf > ?z] : And (?x[rdfs:subPropertyOf > ?y] ?y[rdfs:subPropertyOf > ?z])), Forall ?x ?y ?z1 ?z2 (?z1[?y > ?z2] : And (?x[rdfs:subPropertyOf > ?y] ?z1[?x > ?z2])), Forall ?x (?x[rdfs:subClassOf > rdfs:Resource] : ?x[rdf:type > rdfs:Class]), Forall ?x ?y ?z (?z[rdf:type > ?y] : And (?x[rdfs:subClassOf > ?y] ?z[rdf:type > ?x])), Forall ?x (?x[rdfs:subClassOf > ?x] : ?x[rdf:type > rdfs:Class]), Forall ?x ?y ?z (?x[rdfs:subClassOf > ?z] : And (?x[rdfs:subClassOf > ?y] ?y[rdfs:subClassOf > ?z])), Forall ?x (?x[rdfs:subPropertyOf > rdfs:member] : ?x[rdf:type > rdfs:ContainerMembershipProperty]), Forall ?x (?x[rdfs:subClassOf > rdfs:Literal] : ?x[rdf:type > rdfs:Datatype]), Forall ?x (rif:error : And(?x[rdf:type > rdfs:Literal] ex:illxml(?x))) Forall ?x (?x[rdf:type > rdfs:Literal] : ex:welllit(?x)) 
In the following theorems it is assumed that, in combinations C=<R,{S_{1},...,S_{n}}>, R does not have mentions of rdfs:Resource, S_{1},...,S_{n} do not have mentions of rdfs:Resource beyond triples of the form xxx rdf:type rdfs:Resource, and entailed graphs T and formulas φ do not have mentions of rdfs:Resource.
Theorem An RDFSsatisfiableRIFRDFSsatisfiable listfree RIFRDF combination C=<R,{S_{1},...,S_{n}}> RDFSentailsRIFRDFSentails a generalized RDF graph T if and only if merge({R, R^{RDFS}, tr_{R}(S_{1}), ..., tr_{R}(S_{n})}) entails tr_{Q}(T);
C RDFSentailsRIFRDFSentails a condition formula φ if and only if merge({R, R^{RDFS}, tr_{R}(S_{1}), ..., tr_{R}(S_{n})}) entails φ.
Theorem A listfree RIFRDF combination <R,{S_{1},...,S_{n}}> is RDFSsatisfiableRIFRDFSsatisfiable if and only if merge({R, R^{RDFS}, tr_{R}(S_{1}), ..., tr_{R}(S_{n})}) does not entail rif:error.
It is known that expressive Description Logic languages such as OWL 2 DL cannot be straightforwardly embedded into typical rules languages such as RIF BLD [RIFBLD], because of features such as disjunction and negation.
In this section we consider a subset of OWL 2 DL in RIFOWL DL combinations, namely, the OWL 2 RL profile [OWL2Profiles], and show how reasoning with RIFOWL 2 RL combinations can be reduced to reasoning with RIF.
The embedding of RIFOWL 2 RL combinations is not defined for combinations that include infinite OWL ontologies and for combinations that include ontologies with RDF URI references that are not absolute IRIs or plain literals that are not in the lexical space of the xs:string datatype. In addition, the guard predicate pred:isdt [DTB] must be defined for each datatype used in the combination, where dt is the short name of the datatype.
Since OWL 2 RL includes equality through ObjectMaxCardinality and DataMaxCardinality restrictions, as well as FunctionalObjectProperty UniverseFunctionalObjectProperty, SameIndividual, and HasKey axioms, and there is nontrivial interaction between such equality and the predicates in the RIF rules in the combination, embedding RIFOWL 2 RL combinations into RIF requires equality. Therefore, the embedding presented in this appendix is not in RIF Core, even if the RIF document in the combination is. If the ontologies in the combination do not contain any of the mentioned constructs, the embedding is in Core. Also, it is wellknown that adding equality to a rules language does not increase its expressiveness in the absence of function symbols: one can replace equality = with a new binary predicate symbol, and add rules for reflexivity and the principle of substitutivity (also called the replacement property).
Recall that the semantics of frame formulas in DLdocument formulas is different from the semantics of frame formulas in RIF documents. Nonetheless, DLdocument formulas can be embedded into RIF documents, by translating frame formulas to predicate formulas. The mapping tr is the identity mapping on all RIF formulas, with the exception of frame formulas, as defined in the following table.
In the table, the mapping tr' is an injective function that maps constants to new constants, i.e., constants that are not used in the original document or its vicinity (i.e., entailed or entailing formula). It "generates" a new constant from an existing one.
RIF Construct  Mapping 

Term t  tr(t)=t 
Atomic formula φ that is not a frame formula  tr(φ)=φ 
a[b_{1}>c_{1} ... b_{n}>c_{n}], with n≥2  tr(a[b_{1}>c_{1} ... b_{n}>c_{n}])=And( tr(a[b_{1}>c_{1}]) ... tr(a[b_{n}>c_{n}])) 
a[b > c], where a and c are terms and b ≠ rdf:type is a constant  tr(a[b > c])=tr'(b)(a c) 
a[rdf:type > c], where a is a term and c is a constant  tr(a[rdf:type > c])=tr'(c)(a) 
Exists ?V1 ... ?Vn(φ)  tr(Exists ?V1 ... ?Vn(φ))=Exists ?V1 ... ?Vn(tr(φ)) 
And(φ_{1} ... φ_{n})  tr(And(φ_{1} ... φ_{n}))=And(tr(φ_{1}) ... tr(φ_{n})) 
Or(φ_{1} ... φ_{n})  tr(Or(φ_{1} ... φ_{n}))=Or(tr(φ_{1}) ... tr(φ_{n})) 
φ_{1} : φ_{2}  tr(φ_{1} : φ_{2})=tr(φ_{1}) : tr(φ_{2}) 
Forall ?V1 ... ?Vn(φ)  tr(Forall ?V1 ... ?Vn(φ))=Forall ?V1 ... ?Vn(tr(φ)) 
Group(φ_{1} ... φ_{n})  tr(Group(φ_{1} ... φ_{n}))=Group(tr(φ_{1}) ... tr(φ_{n})) 
Document(directive_{1} ... directive_{n} Γ)  tr(Document(directive_{1} ... directive_{n} Γ))=Document(directive_{1} ... directive_{n} tr(Γ)) 
For the purpose of making statements about this embedding, we define a notion of entailment for DLdocument formulas.
Definition. A RIFBLD DLdocument formula R dlentails a DLcondition φ if for every dlsemantic multistructure Î that is a model of R it holds that TVal_{Î}(φ)=t. ☐
The following lemma establishes faithfulness with respect to entailment of the embedding.
RIFBLD DLdocument formula Lemma A RIFBLD DLdocument formula R dlentails a DLcondition φ if and only if tr(R) entails tr(φ).
Proof. We prove both directions by contradiction: if the entailment does not hold on the one side, we show that it also does not hold on the other.
(=>) Assume tr(R) does not entail tr(φ). This means there is some semantic multistructure I = <TV, DTS, D, D_{ind}, D_{func}, I_{C}, I_{V}, I_{F}, I_{frame}, I_{SF}, I_{sub}, I_{isa}, I_{=}, I_{external}, I_{truth}> that is a model of tr(R), but not of tr(φ).
Consider the dlsemantic multistructure I* = <TV, DTS, D, D_{ind}, D_{func}, I*_{C}, I_{V}, I_{F}, I*_{frame'}, I_{SF}, I_{sub}, I_{isa}, I_{=}, I_{external}, I_{truth}>, with I*_{C} and I*_{frame'} defined as follows: Let t be an element in D such that I_{truth}(t)=t and let f in D be such that I_{truth}(f)=f.Observe that tr(R) and tr(φ) do not include frame formulas.
 for every constant c' used as unary or binary predicate symbol in tr(R) or tr(φ) such that c'=tr'(c) for some constant c, I*_{C}( c')=I_{C}(c); I*_{C}(c*)=I*_{C}(c*) for every other constant c*;
 for every constant c' used as unary predicate symbol in tr(R) or tr(φ) such that c'=tr'(c) for some constant c, and every object k in D_{ind}, if I_{truth}(I_{F}(I_{C}(c'))(k))=t, I*_{frame'}(k)((I_{C}(rdf:type), I_{C}(c))=t,
 for every constant b' used as binary predicate symbol in tr(R) or tr(φ) such that b'=tr'(b) for some constant b, and every pair (k, l) in D_{ind} × D_{ind}, if I_{truth}(I_{F}(I_{C}(b'))(k,l))=t, I*_{frame'}(k)((I_{C}(b),l))=t,
 if I*_{frame'}(k)((b_{1},...,b_{n}))=t and I*_{frame'}(k)((c_{1},...,c_{m}))=t for any two finite bags (b_{1},...,b_{n}) and (c_{1},...,c_{m}), then I*_{frame'}(k)((b_{1},...,b_{n},c_{1},...,c_{m}))=t, and
 I*_{frame'}(b)=f for any other bag b.
To show that I* is a model of R and not of φ, we only need to show that (+) for any frame formula a[b > c] that is a DLcondition, I* is a model of a[b > c] iff I is a model of tr(a[b > c]). This argument straightforwardly extends to the case of frames with multiple b_{i}s and c_{i}s, since in RIF semantic structures the following condition is required to hold: TVal_{I}(a[b_{1}>c_{1} ... b_{n}>c_{n}]) = t if and only if TVal_{I}(a[b_{1}>c_{1}]) = ... = TVal_{I}(a[b_{n}>c_{n}]) = t [RIFBLD].
Consider the case b=rdf:type. Then,
I* is a model of a[b > c] iff I_{truth}(I*_{frame'}(I(a))(I_{C}(rdf:type),I_{C}(c)))=t.
From the definition of I* we obtain
I_{truth}(I*_{frame'}(I(a))(I_{C}(rdf:type),I_{C}(c)))=t iff I*_{frame'}(I(a))(I_{C}(rdf:type),I_{C}(c))=t.
By definition of the embedding, we know that tr'(c) is used as unary predicate symbol in tr(R) or tr(φ). From the definition of I* we obtain
I*_{frame'}(I(a))(I_{C}(rdf:type),I_{C}(c))=t iff I_{truth}(I_{F}(I_{C}(tr'(c)))(I(a)))=t.
Finally, since tr(a[b > c])=tr'(c)(a), we obtain
I_{truth}(I_{F}(I_{C}(tr'(c)))(I(a)))=t iff I is a model of tr(a[b > c]).
From this chain of equivalences follows that I* is a model of a[b > c] iff I is a model of tr(a[b > c]).
The argument for the case b≠rdf:type is analogous, thereby obtaining (+).
(<=) Assume R does not dlentail φ. This means there is some dlsemantic multistructure I = <TV, DTS, D, D_{ind}, D_{func}, I_{C}, I_{V}, I_{F}, I_{frame'}, I_{SF}, I_{sub}, I_{isa}, I_{=}, I_{external}, I_{truth}> that is a model of R, but not of φ. Let B be the set of constant symbols occurring in the frame formulas of the forms a[rdf:type > b] and a[b > c] in R or φ.
Consider the semantic multistructure I* = <TV, DTS, D, D_{ind}, D_{func}, I*_{C}, I_{V}, I*_{F}, I*_{frame}, I_{SF}, I_{sub}, I_{isa}, I_{=}, I_{external}, I_{truth}>. Let t and f in D be such that I_{truth}(t)=t and I_{truth}(f)=f. We define I*_{C}, I*_{frame}, and I*_{F} as follows:Observe that R and φ do not include predicate formulas involving derived constant symbols tr'(b) or tr'(c). The remainder of the proof is analogous to the (=>) direction. ☐
 I*_{C}(tr'(b))=I*_{C}(b) for any b in B ; I*_{C}(c)=I_{C}(c) for any c not in B,
 I*_{frame}(b)=f for any finite bag b of D, and
 I*_{F} is defined as follows:
 for every c in B, given an object k in D_{ind}, if I_{truth}(I_{frame'}(k)((I_{C}(rdf:type), I_{C}(c)))=t, I*_{F}(I*_{C}(tr'(c)))(k)=t; I*_{F}(I*_{C}(tr'(c)))(k')=f for any other k' in D_{ind},
 for every b in B, given a pair (k, l) in D_{ind} × D_{ind}, if I_{truth}(I_{frame}(k)((I_{C}(b),l)))=t, I*_{F}(tr'(b))(k,l)=t;I*_{F}(tr'(b))(k',l')=f for any other pair (k', l') in D_{ind} × D_{ind}, and
 I*_{F}(c')=I_{F}(c') for every other constant c'.
The embedding of OWL 2 RL into RIF BLD has two stages: normalization and embedding.
The OWL 2 syntax is given in terms of a structural model, and there is a functionalstyle syntax that is a serialization of this structural model. For convenience, normalization and embedding in this section are done in terms of the functionalstyle syntax. That is, the normalization mapping takes as input a functionalstyle syntax ontology document and produces a normalized ontology document. The embedding mapping takes as input a normalized ontology document and produces an RIF document. We refer to Section 4.2 of [OWL2Profiles] for the specification of the OWL 2 RL syntax.
Normalization splits the OWL axioms so that the later mapping to RIF of the individual axioms results in rules. Additionally, it simplifies the axioms and removes annotations.
It is assumed that the normalization process is preceded by a simplification process that removes all namespace prefixes, turns all CURIEs and relative IRIs into absolute IRIs, and removes all annotations, import statements, entity declarations, and annotation axioms.
We note here that, strictly speaking, simplified OWL 2 RL ontologies are not OWL 2 RL ontologies in the general case, because certain entity declarations are required (e.g., those distinguishing data from object properties). It is assumed that such entity declarations are present implicitly, i.e., they do not appear explicitly in the simplified ontology, but they are known. We also note that removing import statements in the simplification does not prohibit importing ontologies in practice; since combinations contain sets of ontologies, all imported ontologies may be added to these sets. The normalization mapping tr_{N} takes as input a simplified ontology O and produces an equivalent normalized ontology O'.
The names of variables used in the mapping generally correspond to the names of productions in the OWL 2 RL grammar.
#  Statement  Normalized Statement  Condition on translation 

1  tr_{N}( Ontology( [ ontologyIRI [ versionIRI ] ] 
Ontology( 

2  tr_{N}( SubClassOf(subClassExpression 
tr_{N}(SubClassOf(subClassExpression ...superClassExpression_{1}...)) 

3  tr_{N}( SubClassOf(subClassExpression_{1} ObjectComplementOf(subClassExpression_{1})) ) 
tr_{N}(SubClassOf(ObjectIntersectionof(subClassExpression_{1} subClassExpression_{2}) owl:Nothing)) 

4  tr_{N}(SubClassOf(subClassExpression X)) 
SubClassOf(subClassExpression X) 
X is a superClassExpression that does not contain ObjectIntersectionOf or ObjectComplementOf 
5  tr_{N}( EquivalentClasses( 
tr_{N}(SubClassOf(equivClassExpression_{1} equivClassExpression_{2})) 

6  tr_{N}( DisjointClasses( 
tr_{N}(SubClassOf(ObjectIntersectionOf(subClassExpression_{1} subClassExpression_{2}) owl:Nothing)) ... 

7  tr_{N}( SubObjectPropertyOf( 
SubObjectPropertyOf( 

8  tr_{N}( SubDataPropertyOf( 
SubDataPropertyOf( 

9  tr_{N}( EquivalentObjectProperties( 
tr_{N}(SubObjectPropertyOf(ObjectPropertyExpression_{1} ObjectPropertyExpression_{2})) 

10  tr_{N}( EquivalentDataProperties( 
tr_{N}(SubDataPropertyOf(PropertyExpression_{1} DataPropertyExpression_{2})) 

11  tr_{N}( DisjointObjectProperties( 
DisjointObjectProperties(ObjectPropertyExpression_{1} ObjectPropertyExpression_{2}) 

12  tr_{N}( DisjointDataProperties( 
DisjointDataProperties(DataPropertyExpression_{1} DataPropertyExpression_{2}) 

13  tr_{N}( InverseObjectProperties( 
InverseObjectProperties( 

14  tr_{N}( ObjectPropertyDomain( 
tr_{N}(SubClassOf( 

15  tr_{N}( DataPropertyDomain( 
tr_{N}(SubClassOf( 

16  tr_{N}( ObjectPropertyRange( 
tr_{N}(SubClassOf( 

17  tr_{N}( ObjectPropertyRange( 
tr_{N}(SubClassOf( 
Property is not an inverse property expression 
18  tr_{N}( DataPropertyRange( 
tr_{N}(SubClassOf( 

19  tr_{N}( FunctionalObjectProperty( 
FunctionalObjectProperty( 

20  tr_{N}( FunctionalDataProperty( 
FunctionalDataProperty( 

21  tr_{N}( InverseFunctionalObjectProperty( 
InverseFunctionalObjectProperty( 

22  tr_{N}( IrreflexiveObjectProperty( 
IrreflexiveObjectProperty( 

23  tr_{N}( SymmetricObjectProperty( 
SymmetricObjectProperty( 

24  tr_{N}( AsymmetricObjectProperty( 
AsymmetricObjectProperty( 

25  tr_{N}( TransitiveObjectProperty( 
TransitiveObjectProperty( 

26  tr_{N}( DatatypeDefinition( ... ) 
DatatypeDefinition( ... )  
27  tr_{N}( HasKey( ... ) 
HasKey( ... )  
28  tr_{N}( SameIndividual( 
SameIndividual(Individual_{1} Individual_{2}) 

29  tr_{N}( DifferentIndividuals( 
DifferentIndividuals(Individual_{1} Individual_{2})/tt> 

30  tr_{N}( ClassAssertion( 
SubClassOf(ObjectOneOf( Individual ) superClassExpression )  
31  tr_{N}( ObjectPropertyAssertion( 
SubClassOf(ObjectOneOf( source ) ObjectHasValue(ObjectPropertyExpression target) )  
32  tr_{N}( NegativeObjectPropertyAssertion( 
SubClassOf(ObjectOneOf( source ) ObjectComplementOf(ObjectHasValue(ObjectPropertyExpression target) ) )  
33  tr_{N}( DataPropertyAssertion( 
SubClassOf(ObjectOneOf( source ) DataHasValue(DataProperty target) )  
34  tr_{N}( NegativeDataPropertyAssertion( 
SubClassOf(ObjectOneOf( source ) ObjectComplementOf(DataHasValue(DataProperty target) ) ) 
We note that normalized OWL 2 RL ontologies are not necessarily OWL 2 RL ontologies, since owl:Thing may appear in subclass expressions, as a result of the transformation of DataPropertyRange axioms.
The following lemma establishes the fact that, for the purpose of entailment, the ontologies in a combination may be replaced with their normalization.
Normalization Lemma Given a combination C=<R,{O_{1},...,O_{n}}>, where O_{1},...,O_{n} are simplified OWL 2 RL ontologies that do not import ontologies, C OWLRIFOWL DLentails φ iff C'=<R,{tr_{N}(O_{1}),...,tr_{N}(O_{n})}> OWLRIFOWL DLentails φ.
We now proceed with the embedding of normalized OWL 2 RL ontologies into RIF DLdocument formulas. The embedding function tr_{O} takes as input a normalized OWL 2 RL ontology and returns a RIFBLD DLdocument formula. The embeddings of IRIs and literals is as defined in the Section Embedding Symbols.
When we speak about class IDs and datatype IDs we mean IRIs used as identifiers for classes, respectively datatypes. By the syntactic restrictions on OWL ontologies, these sets are disjoint. Similarly, we speak about data, object, and annotation property IDs when talking about IRIs used as identifiers for data, object, respectively annotation properties. These sets of property identifiers are mutually disjoint for any OWL ontology.
#  Normalized OWL  RIF RIFBLD DLdocument formula  Condition on translation 

1  tr_{O}( Ontology( 
Document(Group( tr_{O}(axiom_{1}) 

2  tr_{O}( SubClassOf(subClassExpression superClassExpression) 
tr_{O}(subClassExpression,superClassExpression,?x) 

3  tr_{O}(subClassExpression, [ObjectData]AllValuesFrom(property_{1} ...[ObjectData]AllValuesFrom(property_{n} X) ...),?x) 
Forall ?x ?y_{1} ... ?y_{n} (tr_{O}(X, ?y_{n}) : And( 
X is not an [ObjectData]AllValuesFrom or [ObjectData]MaxCardinality expression. If n=0, ?y_{n} is ?x. 
4  tr_{O}(subClassExpression, [ObjectData]AllValuesFrom(property_{1} ...AllValuesFrom(property_{n} MaxCardinality(0 PropertyExpression ClassExpression) ...),?x) 
Forall ?x ?y_{1} ... ?y_{n} ?z (rif:error) : And( 

5  tr_{O}(subClassExpression, [ObjectData]AllValuesFrom(property_{1} ...[ObjectData]AllValuesFrom(property_{n} [ObjectData]MaxCardinality(1 PropertyExpression ClassExpression) ...),?x) 
Forall ?x ?y_{1} ... ?y_{n} ?z_{1} ?z_{2} (?z1=?z2) : And( 

6  tr_{O}(A,?x) 
?x[rdf:type > tr(A)] 
A is a Class or Datatype; x is a variable name 
7  tr_{O}([ObjectData]IntersectionOf(ClassExpression_{1} ... ClassExpression_{n}), ?x) 
And(tr_{O}(ClassExpression_{1}, ?x) ... tr_{O}(ClassExpression_{n}, ?x)) 
x is a variable name 
8  tr_{O}(ObjectUnionOf(ClassExpression_{1} ... ClassExpression_{n}), ?x) 
Or(tr_{O}(ClassExpression_{1}, ?x) ... tr_{O}(ClassExpression_{n}, ?x)) 
x is a variable name 
9  tr_{O}([ObjectData]OneOf(Individual_{1} ... Individual_{n}), ?x) 
Or( ?x = tr(Individual_{1}) ... ?x = tr(Individual_{n})) 
x is a variable name 
10  tr_{O}([ObjectData]SomeValuesFrom(PropertyExpression ClassExpression)), ?x) 
Exists ?y(And( tr_{O}(PropertyExpression, ?x, ?y)] tr_{O}(ClassExpression, ?y) )) 
x is a variable name 
11  tr_{O}(X, ?x, ?y) 
?x[tr(X) > ?y] 
X is a Property; x, y are variable names 
12  tr_{O}(ObjectInverseOf(X), ?x, ?y) 
?y[tr(Property) > ?x] 
X is a Property; x, y are variable names 
13  tr_{O}([ObjectData]HasValue(PropertyExpression value), ?x) 
tr_{O}(PropertyExpression, ?x, tr(value) 
x is a variable name 
14  tr_{O}( SubObjectPropertyOf(ObjectPropertyChain(PropertyExpression_{1} ... PropertyExpression_{m}) PropertyExpression_{0}) 
Forall ?x ?y_{1} ... ?y_{m} (tr_{O}(PropertyExpression_{1}, ?x, ?y_{m}) : And( 

15  tr_{O}( Sub[ObjectData]PropertyOf(PropertyExpression_{1} PropertyExpression_{2}) 
Forall ?x ?y (tr_{O}(PropertyExpression_{2}, ?x, ?y) : tr_{O}(PropertyExpression_{1}, ?x, ?y)) 
PropertyExpression_{1} contains no mention of ObjectPropertyChain 
16  tr_{O}( Disjoint[ObjectData]Properties(PropertyExpression_{1} PropertyExpression_{2}) 
Forall ?x ?y (rif:error : And(tr_{O}(PropertyExpression_{1}, ?x, ?y) tr_{O}(PropertyExpression_{2}, ?x, ?y))) 

17  tr_{O}( InverseObjectProperties(PropertyExpression_{1} PropertyExpression_{2}) 
Forall ?x ?y (tr_{O}(PropertyExpression_{2}, ?y, ?x) : tr_{O}(PropertyExpression_{1}, ?x, ?y)) 

18  tr_{O}( Functional[ObjectData]Property(PropertyExpression) 
Forall ?x ?y_{1} ?y_{2} (?y_{1}=?y_{2} : And(tr_{O}(PropertyExpression, ?x, ?y_{1}) tr_{O}(PropertyExpression, ?x, ?y_{2}))) 

19  tr_{O}( InverseFunctional[ObjectData]Property(PropertyExpression) 
Forall ?x_{1} ?x_{2} ?y (?x_{1}=?x_{2} : And(tr_{O}(PropertyExpression, ?x_{1}, ?y) tr_{O}(PropertyExpression, ?x_{2}, ?y))) 

20  tr_{O}( IrreflexiveObjectProperty(PropertyExpression) 
Forall ?x (rif:error : tr_{O}(PropertyExpression, ?x, ?x)) 

21  tr_{O}( SymmetricObjectProperty(PropertyExpression) 
Forall ?x ?y (tr_{O}(PropertyExpression, ?y, ?x) : tr_{O}(PropertyExpression, ?x, ?y)) 

22  tr_{O}( AsymmetricObjectProperty(PropertyExpression) 
Forall ?x ?y (rif:error : And(tr_{O}(PropertyExpression, ?x, ?y) tr_{O}(PropertyExpression, ?y, ?x))) 

23  tr_{O}( TransitiveObjectProperty(PropertyExpression) 
Forall ?x ?y ?z (tr_{O}(PropertyExpression, ?x, ?z) : And(tr_{O}(PropertyExpression, ?x, ?y) tr_{O}(PropertyExpression, ?y, ?z))) 

24  tr_{O}( DatatypeDefinition(datatypeIRI DataRange) 
Forall ?x (?x[rdf:type > tr(datatypeIRI)] : tr_{O}(DataRange, ?x)) 

25  tr_{O}( HasKey(subClassExpression PropertyExpression_{1} ... PropertyExpression_{m}) 
Forall ?x ?y ?z_{1} ... ?z_{m} (?x=?y : And(tr_{O}(subClassExpression, ?x) tr_{O}(subClassExpression, ?y) tr_{O}(PropertyExpression_{1}, ?x, ?z_{1}) ... tr_{O}(PropertyExpression_{1}, ?x, ?z_{m}) tr_{O}(PropertyExpression_{1}, ?y, ?z_{1}) ... tr_{O}(PropertyExpression_{1}, ?y, ?z_{m}))) 

26  tr_{O}( SameIndividual(Individual_{1} Individual_{2}) 
tr(Individual_{1})=tr(Individual_{2}) 

27  tr_{O}( DifferentIndividuals(Individual_{1} Individual_{2}) 
rif:error : tr(Individual_{1})=tr(Individual_{2}) 
Besides the embedding in the previous table, we also need an axiomatization of some of the aspects of the OWL DL semantics, e.g., separation between individual and datatype domains. This axiomatization is defined relative to an OWL vocabulary V, which includes all welltyped literals used in the rules, and a datatype map D, which includes all considered datatypes. In the table, for a given datatype d, L2V(d) is the lexicaltovalue mapping of d.
R^{OWL}(V)  =  merge({ (i) (Forall ?x (rif:error : ?x[rdf:type > owl:Nothing]), 
We call an OWL 2 RL ontology O normalized if it is the same as its normalization, i.e., O=tr_{N}(O).
The following lemma establishes faithfulness of the embedding.
Normalized Combination Embedding Lemma Given a datatype map D conforming with T, a RIFOWL DLcombination C=<R,{O_{1},...,O_{n}}>, where {O_{1},...,O_{n}} is an importsclosed set of normalized OWL 2 RL ontologies with vocabulary V, OWLRIFOWL DLentails a DLcondition φ with respect to D iff merge({R, R^{OWLDL}(V), tr_{O}(O_{1}), ..., tr_{O}(O_{n})}) dlentails φ.
The following theorems establish faithfulness of the full embedding of RIFOWL 2 RL combinations into RIF.
Theorem Given a datatype map D conforming with T, a RIFOWL DLcombination C=<R,{O_{1},...,O_{n}}>, where {O_{1},...,O_{n}} is an importsclosed set of OWL 2 RL ontologies with vocabulary V, OWLRIFOWL DLentails a DLcondition formula φ with respect to D iff tr(merge({R, R^{OWLDL}(V), tr_{O}(tr_{N}(O_{1})), ..., tr_{O}(tr_{N}(O_{n}))})) entails tr(φ).
Theorem Given a datatype map D conforming with T, a RIFOWLDLcombination <R,{O_{1},...,O_{n}}>, where {O_{1},...,O_{n}} is an importsclosed set of OWL 2 RL ontologies with vocabulary V, is owldlsatisfiable with respect to D iff tr(merge({R, R^{OWLDL}(V), tr_{O}(tr_{N}(O_{1})), ..., tr_{O}(tr_{N}(O_{n}))}) does not entail rif:error.
Changes since the 303 July 20082009 Working Draft.
ErrorIn the definition of dlsemanticstructurecommonRIFRDFinterpretation, "RDF interpretation" was corrected; the previous version of the definition did not disallow class and property identifiers to be mapped in the individual domain. Some missing CURIE prefixes were added in the Appendix, references to rif:text have been updated to rdf:PlainLiteral, in linereplaced with "Simple interpretation". The change in [ RIFDTB ], and some typos were corrected in the main text. The embeddings of the Simple, RDF, and RDFS entailment regimes have been updated so that they are now in RIF Core. Itoriginal reference to "RDF interpretation" was previously undefined what happens when importing graphs with the OWL DL profile, but the document is not a DLdocument formula oran error.
The graphs are not representations of OWL DL ontologies. OWL compatibility is now defined for OWL 2, which extendsnotions RDFSmodel, OWL 1. Since annotationsDLentailment, etc. have no semantics in OWL 2 DL, the notion of owldlannotationentailment is rendered meaningless, and has thusbeen removed.renamed to RIFRDFSmodel, RIFOWL DLentailment, etc.
RDF URI References: There are certain RDF URI references that are not IRIs (e.g., those containing spaces). It is possible to use such RDF URI references in RDF graphs that are combined with RIF rules. However, such URI references cannot be represented in RIF rules and their use in RDF is discouraged.
Generalized RDF graphs: Standard RDF graphs, as defined in [RDFConcepts], do not allow the use of literals in subject and predicate positions and blank nodes in predicate positions. The RDF Core working group has listed two issues questioning the restrictions that literals may not occur in subject and blank nodes may not occur in predicate positions in triples. Anticipating lifting of these restrictions in a possible future version of RDF, we use the more liberal notion of generalized RDF graph. We note that the definitions of interpretations, models, and entailment in the RDF semantics document [RDFSemantics] also apply to such generalized RDF graphs.
We note that every standard RDF graph is a generalized RDF graph. Therefore, our definition of combinations applies to standard RDF graphs as well.
We note also that the notion of generalized RDF graphs is more liberal than the notion of RDF graphs used by SPARQL; generalized RDF graphs additionally allow blank nodes and literals in predicate positions.