Unification with the occurs check

Source: The Art of Prolog

Program source code: unification_2.pl

/*
   unify(Term1,Term2) :- Term1 and Term2 are unified with the occurs check.
*/
   unify(X,Y) :-
	var(X), var(Y), X=Y.
   unify(X,Y) :-
	var(X), nonvar(Y), not_occurs_in(X,Y), X=Y.
   unify(X,Y) :-
	var(Y), nonvar(X), not_occurs_in(Y,X), Y=X.
   unify(X,Y) :-
	nonvar(X), nonvar(Y), constant(X), constant(Y), X=Y.
   unify(X,Y) :-
	nonvar(X), nonvar(Y), compound(X), compound(Y), term_unify(X,Y).
 
   not_occurs_in(X,Y) :-
	var(Y), X \== Y.
   not_occurs_in(X,Y) :- 
	nonvar(Y), constant(Y).
   not_occurs_in(X,Y) :-
	nonvar(Y), compound(Y), functor(Y,F,N), not_occurs_in(N,X,Y).
 
   not_occurs_in(N,X,Y) :-
	N > 0, arg(N,Y,Arg), not_occurs_in(X,Arg), N1 is N-1, 
			not_occurs_in(N1,X,Y).
   not_occurs_in(0,X,Y).
 
   term_unify(X,Y) :-
	functor(X,F,N), functor(Y,F,N), unify_args(N,X,Y).
 
   unify_args(N,X,Y) :-
	N > 0, unify_arg(N,X,Y), N1 is N-1, unify_args(N1,X,Y).
   unify_args(0,X,Y).
 
   unify_arg(N,X,Y) :-
	arg(N,X,ArgX), arg(N,Y,ArgY), unify(ArgX,ArgY).
 
%  Program 10.6    Unification with the occurs check