- rdf_nth0(?Index, +RDFList, ?X) is nondet
- rdf_nth1(?Index, +RDFList, ?X) is nondet
- True when X is the Index-th element (0-based or 1-based) of
RDFList. This predicate is deterministic if Index is given and
the list has no multiple rdf:first or rdf:rest values.