A query is composed of an RDF dataset, or references to the graph or
graphs against which the query will be executed; and the query itself -- in this
case, the query is a string conforming to a grammar @@.
@@ quote the spec...
An rdf-dataset is a set of URIs, zero or one of which is the default
graph (which might also be called the anonymous graph, since it cannot be named);
and zero or any number of named graphs.
The type for serializaing query results, either as VBR XML or
RDF/XML.