code: function(p, q) {
                  var v = this.getVertex(), pv = Vector.fromPoints(p.getPoint(), v), vq = Vector.fromPoints(v, q.getPoint());
                  vq = pv.normalize(this.isSymReflaction() ? pv.length() : vq.length());
                  q.setPoint(v.x + vq.x, v.y + vq.y);
                  return this;
              }
docs: @private
  
  根据前方控制点或后方控制点更新另一方