int binary_gcd (int x, int y) {
  if (x == y) { return x; }
  if (x == 0) { return y; }
  if (y == 0) { return x; }
  if ((~x [&] 1) == 1) {
    if ((y [&] 1) == 1) {
      return binary_gcd(x >> 1, y);
    }
    else {
      return binary_gcd(x >> 1, y >> 1) << 1;
    }
  }
  if ((~y [&] 1) == 1) {
    return binary_gcd(x, y >> 1);
  }
  if (x > y) {
    return binary_gcd((x - y) >> 1, y);
  }
  return binary_gcd((y - x) >> 1, x);
}

int program (int argc, string[] argv) {
  var x = 21;
  var y = 15;
  return binary_gcd(x, y);
}