public class GCD {
    /*@
      @ requires a>=0 && b>=0;
      @ ensures \result >= 0;
      @ ensures (\exists int q; \result*q == a) &&
      @         (\exists int q; \result*q == b) &&
      @     (\forall int c;
      @         (\exists int q; c*q == b) && (\exists int q; c*q == a);
      @         (\exists int q; c*q == \result));
      @*/
    public static int gcd(int a, int b) {
	/*@ loop_invariant a >= 0 && b>=0 &&
	  @    (\forall int d;
	  @         (\exists int q; \old(a) == q*d) && (\exists int q; \old(b) ==q*d)
	  @      <==>
	  @         (\exists int q; a == q*d) && (\exists int q; b ==q*d));
	  @ assignable a,b;
	  @ decreases a+b;
	  @*/
	while (a != 0 && b != 0) {
	    if (a > b)
		a = a - b;
	    else
		b = b - a;
	}
	return (a > b) ? a : b;
    }
}