Write a Prolog predicate, subtract(X, Y, Z), which is true if Z
represents the difference between X and Y, assuming X is greater than or
equal to Y. If X is less than Y, then this predicate is undefined.
?- subtract(s(s(s(s(0)))), s(s(0)), Z).
Z = s(s(0))

**Answer:**

subtract(X, 0, X).
subtract(s(X), s(Y), Z) :-
subtract(X, Y, Z).