Puzzling OO design problem

[Michael Spencer]

George Sakkis wrote:
> 
> Nice try, but ideally all boilerplate classes would rather be avoided
> (at least being written explicitly). 

It depends on how much magic you are prepared to accept; this goal is somewhat 
in conflict with the next one...

Also, it is not obvious in your
> solution why and which placeholder classes have to be written (like
> World2.Movable) and which do not. 

It is systematic, if not obvious.  You need place holder classes only if you 
need to propagate new methods within a given world.  More magic would make this 
even less obvious.

By the way, my current working
> solution involves copying and pasting verbatim these classes :-) 


It appears to be basically the same as mine except you spell out the previous 
version explicitly (not that that's bad!)

Have you considered a 'macro' solution composing source?  If I were handling so 
many versions, I would want a complete class definition for each version rather 
than having to scan many sources for each implementation.

Michael