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Write a Prolog predicate, minus1(X,Y), which is true if X is the successor of Y, i.e. if X = Y+1, or Y=X-1. Naturally, this predicate will fail if X is 0.
?- minus1(s(s(0)), Y). Y = s(0) ?- minus1(0, Y). false.
Answer:
minus1(s(X), X).