说明：  正方形四个顶点上各有一个机器人，它们同时以相同的速度开始运动，且每个机器人以前一个机器人为目标运动。试求：机器人的轨迹！
(Each of the four vertices of a square robot, they also began to sport the same speed and each robot is a robot before the target motion. Find: Trajectory!)