Availability:C-language interface function
?t, functor_t f)If t is a compound term with the given functor, just succeed.
If it is unbound, create a term and bind the variable, else fail. Note
that this function does not create a term if the argument is already
instantiated. If f is a functor with arity 0, t is
unified with an atom. See also PL_unify_compound().