def on_exit_stage(self):
pass<EOL>
Give the stage a chance to react before it is stopped and the next 
stage is started.

You can define the next stage by setting the Stage.successor attribute.  
If the successor is static, you can just set it in the constructor.  
But if it will differ depending on the context, this method may be a 
good place to calculate it because it is called only once and just 
before the theater queries for the successor.