/* gcd(positive no , positive no , answer) */

gcd(X,Y,D)	:-  	X=:=Y,
			D is X.

gcd(X,Y,D)	:-	X<Y,
			Y1 is Y-X,
			gcd(X,Y1,D).

gcd(X,Y,D)	:-	X>Y,
			X1 is X-Y,
			gcd(X1,Y,D).