Thanks for the feedback. Regarding your final comment, the existing RIF specification for externals *is* a general and extensible method for attaching procedures, which could be builtins or be defined using existing programming languages. See FLD Section 2.4 item number 8:
[External] terms are used for representing built-in functions and predicates as well as "procedurally attached" terms or predicates, which might exist in various rule-based systems, but are not specified by RIF.
DTB is a list of externals that are *required* for interoperability, but does not preclude defining others.
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-The RIF WG
Tom Gordon wrote: > The time is ripe for a W3C standard for rules and RIF looks to me to be > a very good proposal, building on the experience of previous work on > SWRL and RuleML, among other initiatives. I particularly like its > modular structure and its extensibility using FLD. It remains to be > seen whether FLD will be expressive enough for requirements in the legal > domain, for defining a dialect capable of modeling legislation in an > "isomorphic" way, which is important for both validating and maintaining > the models. In a three year European project, ESTRELLA, which ended in > 2008, we developed a rule interchange language for models of > legislation, called the Legal Knowledge Interchange Format (LKIF) > expressly for this purpose. We may have built LKIF on top of RIF had it > been available at the time. It may be an interesting task to see if > LKIF could be reconstructed as a RIF dialect using FLD. > > One nitpick: RIF, like SWRL before it, define a bunch of "builtin" > predicate and function symbols. I would have much preferred a more > general and extensible method for attaching procedures, defined using > existing programming langauges.