You may assume that the user enters convex polygons (that is,
you need not check them before/while doing the clipping).
<p>

Some links to billiards references:

<ul>

<li> <a href="http://serendip.brynmawr.edu/chaos/home.html">java
simulation without pillars</a>

<li>


<a href="http://www.sciencenews.org/sn_arc97/3_1_97/mathland.htm">
Science News article</a>

<li>
<a href="http://www.mpipks-dresden.mpg.de/~jwiersig/interest/polygons/polygons.html">
Billiards, chaos, and quantum mechanics</a>



</ul>