function isHook(pt1, pt2, pt3, pt4, pt5, pt6) {
  if (pt1.on || pt2.on || !pt3.on || !pt4.on || pt5.on || pt6.on) return false;
  if (!(Math.abs(pt3.y - pt2.y) <= 1) || !(Math.abs(pt4.y - pt3.y) <= 1) 
  || !(Math.abs(pt5.y - pt4.y) <= 1) || !(Math.abs(pt5.y - pt4.y) <= 1)) 
    return false;
  if (pt1.y > pt2.y && pt6.y > pt5.y) return true;
}
Detect a horizontal hook
@param { ControlPoint } pt1 
@param { ControlPoint } pt2 
@param { ControlPoint } pt3 
@param { ControlPoint } pt4 
@param { ControlPoint } pt5 
@param { ControlPoint } pt6 
@returns { boolean }