I was wondering about the possibility of being able to temporarily replace 
a class with another (to be able to easily mock a class like File for 
example).

I found an implementation of pose_as on a posting from this list 
describing an implementation for pose_as:
http://www.ruby-talk.org/cgi-bin/vframe.rb/ruby/ruby-talk/20008?19808-25362

Matz had followed up with showing that you can do:
	OldClass = NewClass

This doesn't work within a set_up method (Dynamic Constant Assignment) and 
also doesn't let me put the old one back in the teardown.

The pose_as implementation seems to have a problem or two, and before I 
try and debug it, is there:
	a) a good implementation somewhere, or
	b) a similarly simple way to OldClass=NewClass that I can use in a 
dynamic context

Thanks,

-Tom