小孩和玩具

代码说明：

  设一个小孩在平面上沿着某曲线行走，此曲线由时间参数曲线确定。假设小孩借助于一个长度为 的硬木棒拖着或推着一个玩具，试求玩具的平面轨迹。(set up a child in a plane curve walk along this curve by the time parameter curve. Using a hypothetical child for the length of the hard wooden dragging or pushing a toy, toys trial for the plane trajectory.)