// file:    gcd.rex
// author:  R. Keller
// purpose: greatest-common divisor
//          (Euclid's algorithm)

gcd(x, 0) => x;
gcd(0, y) => y;
gcd(x, y) => x <= y ? gcd(y%x, x) : gcd(x%y, y);

// Rationale:  If one number is 0, the gcd is the other number 
// Otherwise neither is 0, so replacing the larger number by it divided
// by the smaller gives a pair of numbers with the same gcd (This needs
// to be proved.

test() = [gcd(36, 28), gcd(0, 99), gcd(99, 0), gcd(1234567, 765438)];