% 2.07 (**) Determine the greatest common divisor of two positive integers.
% gcd(X,Y,G) :- G is the greatest common divisor of X and Y
% (integer, integer, integer) (+,+,?)
gcd(X,0,X) :- X > 0.
gcd(X,Y,G) :- Y > 0, Z is X mod Y, gcd(Y,Z,G).
% Declare gcd as an arithmetic function; so you can use it
% like this: ?- G is gcd(36,63).
:- arithmetic_function(gcd/2).