====== Occurs ======
{{tag>terms}}
===== Description =====
Occurs in


**Source**:  The Art of Prolog
===== Download =====
Program source code: {{occurs.pl}}

===== Listing =====
<code prolog>
/*

   occurs_in(Sub,Term) :- 

	Sub is a subterm of the (possibly non-ground term) Term.

*/

% a:  Using ==



   occurs_in(X,Term) :- subterm(Sub,Term), X == Sub.



% b:  Using freeze    $$$$  freeze is not in Standard Prolog



   occurs_in(X,_) :- freeze(X,Xf), freeze(_,Termf), subterm(Xf,Termf).



/*

   subterm(Sub,Term) :- Sub is a subterm of the ground term Term.

*/

     subterm(Term,Term).

     subterm(Sub,Term) :- 

        compound(Term), functor(Term,_,N), subterm(N,Sub,Term).



     subterm(N,Sub,Term) :- 

        arg(N,Term,Arg), subterm(Sub,Arg).

     subterm(N,Sub,Term) :- 

        N > 1, N1 is N-1, subterm(N1,Sub,Term).



%  Program 

0.7    Occurs in

</code>
===== Comments =====