functor(T,F,N)

The  principal  functor of term T has name F and arity N, where F is either
an  atom  or,  provided  N  is  0, an integer.  Initially, either T must be
instantiated  to  a  non-variable,  or  F  and  N  must be instantiated to,
respectively,  either  an atom and a non-negative integer or an integer and
0.   If  these conditions are not satisfied, an error message is given.  In
the case where T is initially instantiated to a variable, the result of the
call  is  to  instantiate  T  to the most general term having the principal
functor indicated.