% Implementation of nondestructive arithmetic :- consult(gv). add(X, Y, S) :- value_of(X, XVal), value_of(Y, YVal), declare(S, XVal), add(S, YVal). sub(X, Y, D) :- value_of(X, XVal), value_of(Y, YVal), declare(D, XVal), sub(D, YVal). mul(X, Y, P) :- value_of(X, XVal), value_of(Y, YVal), Product is XVal * YVal, declare(P, Product). div(X, Y, Q) :- value_of(X, XVal), value_of(Y, YVal), Quotient is XVal / YVal, declare(Q, Quotient). exp(X, Y, P) :- value_of(X, XVal), value_of(Y, YVal), exponential(XVal, YVal, Product), declare(P, Product). exponential(_, 0, 1) :- !. exponential(X, Y, P) :- YM1 is Y - 1, exponential(X, YM1, Partial), P is X * Partial.