Re: noverlay branch

[Stefan Monnier]

>  3) Improve quality of comments in the new code.  Personally, I find the
>     algorithms quite subtle and quite a bit more complex than what you
>     find on, say, https://en.wikipedia.org/wiki/Interval_tree or the
>     Cormen et al. Introduction to Algorithms Book.  I think I pieced
>     most of it together but it took a lot of effort.  At top of mind is
>     looking at the interval_node.visited flag and figure out how that
>     flag is used, then describe the algorithm in detail.  It isn't clear
>     to me how that flag gets set/cleared.  Best case: doing so proves me
>     wrong on point (1).

I can explain the `visited` bit (and I added a comment about it in the
code).

What I'm less clear about is the use of the `null` sentinel node.
It seems that this node is sometimes modified (e.g. its color changed
from black to red or vice versa, and maybe also its `parent` field),
even though it's pointed to from lots of different nodes, so any help
documenting the way it works (or why the value in those fields doesn't
matter) would be welcome.


        Stefan