gcd (U, 0, U). gcd (U, V, W) : not (V = 0), R is U mod V, gcd (V, R, W). append([], Y, Y). append([A B], Y, [AW]) :- append (B, Y, W). reverse ([], []). reverse ([HT], L) :- reverse (T, L1), append (L1, [H], L). Figure 4.1 Prolog clauses for gcd, append, and reverse (a) gcd (15, 10, X). (b) append (X, Y, [1, 2]). gcd (U, 0, U). gcd (U, V, W) : not (V = 0), R is U mod V, gcd (V, R, W). append([], Y, Y). append([A B], Y, [AW]) :- append (B, Y, W). reverse ([], []). reverse ([HT], L) :- reverse (T, L1), append (L1, [H], L). Figure 4.1 Prolog clauses for gcd, append, and reverse