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

> About mixins why not use the given functionality of (mixin) of class.ss? 
> I suppose its just a simple macro but why reinvent the wheel. 
> http://download.plt-scheme.org/doc/301/html/mzlib/mzlib-Z-H-4.html#node_sec_4.6

Hi Jon,

I wanted to, but use of the MIXIN macro requires knowledge about 
interfaces; I can't use it unless I first add a section on them.

But I do agree with Ryan Culpepper that I should include interfaces, 
though, so I'll cook something up soon, and later I can revisit MIXIN in 
the tutorial.


> Also would you mind adding something on generic field accessors? I dont 
> think it was completely obvious to me in the beginning why they were 
> useful.

Do you mean the GENERIC form?  As far as I understand so far, they allow 
us to avoid an expensive dynamic name->method lookup.  But they don't seem 
to add anything other than a performance optimization boost.  That limited 
applicability makes me want to skip over them for the moment.


Thanks again to everyone for the suggestions!