static int div(int x, int y){
    // require x > 0 && y > 0;   

    int rest = x;
    int quotient = 0;
    
    while (rest >= y) {
      // invariant rest + quotient * y = x;   
      // System.out.println(rest + quotient * y == x);   
      rest = rest - y;
      quotient = quotient + 1;
    }
    return (quotient);
    // ensure  rest + quotient * y = x && rest < y && rest >= 0;   
  } // end div