My pen pal Peter came to Beijing 3 months ago. There days he  小题1: (learn) Chinese in a language school. He likes travelling. And he  小题2:  already      (visit) many great places of interest in our country. Next month, he  小题3:  (fly) to Harbin for the summer vocation. I am sure he will have a good time.

Weiler-Atherton多边形裁减算法可以处理任何非自相交多边形。