/* Code of Figure 1.6, page 19 from
   Kenneth C. Louden, Programming Languages
   Principles and Practice 2nd Edition
   Copyright (C) Brooks-Cole/ITP, 2003
*/

gcd(U, V, U) :- V = 0 .
gcd(U, V, X) :- not(V = 0),
                Y is U mod V,
                gcd(V, Y, X).