%Source: https://sites.google.com/site/prologsite/prolog-problems
%query:gcd(g,g,f).
% 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).