[plt-scheme] Added section in my oop tutorial for inheritance and mixins; comments?

Hi everyone,

Sorry for being so late about this, but I restarted work on getting that 
"class.ss tutorial for Java programmers" thing more fleshed out.  One of 
the requests I received was to add a section on mixins, so I wrote 
something up that shows where they can be useful.  I also fleshed out the 
section on inheritance a little more to explain to myself some of the 
differences between inherit vs. send, just from an early error-trapping 
perspective.

Would anyone like to suggest improvements and corrections to this? 
Here's a link to the revised tutorial:

     http://hkn.eecs.berkeley.edu/~dyoo/plt/oop-notes.text

I'm writing this stuff as I'm learning it, so this is also an implicit 
request for correctness verification.  *grin*


Best of wishes!