_Rb_tree + allocator observations

[Paolo Carlini]

Matt Austern wrote:

> Dunno if Nathan has something to add, but I do.  I wrote that comment,
> and the code it applies to.  I wrote it for reasons that are no longer
> relevant.  (I was supporting two versions of the allocator interface,
> one of which nobody cares about anymore, and I was supporting the SGI
> compiler, which at the time didn't optimize empty base classes.)

Thanks for your feedback: therefore my first impression was basically 
right...

> It's possible to make *huge* simplifications in that code.  I should
> get around to doing that.

It would be great!

Paolo.