public class GCD2 {

    public static int gcd(int olda, int oldb)
    /*:
      requires "0 <= olda & 0 <= oldb"
      ensures "0 <= result &
               (EX q. result*q = olda) & (EX q. result * q = oldb) &
	       (ALL c. (EX q. c*q = olda) & (EX q. c*q = oldb)
	               --> (EX q. c*q = result))"
    */
    {
	int a = olda;
	int b = oldb;
	while /*: inv  "0 <= a & 0 <= b &
		        (ALL c.     (EX q. c*q = olda) & (EX q. c*q = oldb)
			        <-> (EX q. c*q =    a) & (EX q. c*q =    b))"
	       */ (a != 0 && b != 0) {
	    if (a > b)
		a = a - b;
	    else
		b = b - a;
	}
	return a > b ? a : b;
    }
}