Ignore the mess / work in progress
Grouping by contiguous siblings -- it works, but I'd like to find a
better way to do it than using as many for loops as I do at present.
I've written down a set of "mptt axioms" that I believe to be useful
when writing code to manipulate mptt trees. Once I'm sure they're
correct, I'll put them somewhere accessible... for now though, they've
already started helping me think about the code in a better way.
Minor cleanup
Revert "I've written down a set of "mptt axioms" that I believe to be useful"

This reverts commit 404524e767facc4ca30104f9eb1c05f4b77af185.

Conflicts:
	mptt/managers.py
Overwrote my last changes by accident... this file corrects that mistake