anchor.diff:     * Stop the acceleration, both speed up and slow down.
      */
     stopSpeedChange() {
  -    _accelerationDirection = 0;
  +    _accelerationDirection = _NO_ACCELERATION_DIRECTION;
     }
   
   
  
positive.code:    */
    slowDown() {
      if (_speedUnderEqualMinSpeed() && _accelerationDirection == -1) {
        _accelerationDirection = 0;
      } else {
        _accelerationDirection = -1;
      }
    }
  
  
negative.code:    */
    speedUP() {
      if (_speedOverEqualMaxSpeed() && _accelerationDirection == 1) {
        _accelerationDirection = _NO_ACCELERATION_DIRECTION;
      } else {
        _accelerationDirection = _SPEED_UP_DIRECTION;
      }
    }
  
  