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instantiation_error()if Term is unbound and Name/Arity is insufficiently instantiated.
SWI-Prolog also supports terms with arity 0, as in
section 5). Such terms
must be processed using
The predicate functor/3
=../2 raise a
when faced with these terms. Without this precaution a round trip
of a term with arity 0 over functor/3
would create an atom.