:- use_module(library(chr)).

:- chr_constraint gcd/1.

gcd(0) <=> true.
gcd(N) \ gcd(M) <=> N=<M | L is M-N, gcd(L).

/***********************
Here are a few test queries.

?- gcd(2),gcd(3).

?- X is 37*11*11*7*3, Y is 11*7*5*3, Z is 37*11*5, gcd(X), gcd(Y), gcd(Z).

**************************/