RFC on range design for D2

[Benji Smith]

Bill Baxter wrote:
> Ok, but I have yet to hear an actual use case that demands blazing
> fast iteration both forwards and backwards.  In your shuffling video
> there's no way moving the iterator back and forth is going to be the
> bottleneck.  In my undo/redo stack example it is also far from being
> on the critical path.    I think it goes back to the fact that going
> back and forth randomly isn't a property of many algorithms.  In all
> the examples I can think of it's more a property of how humans
> interact with data.  And humans are slow compared to how long it takes
> to update a few extra values.

Oh!! I thought of one!!

Parsers & regex engines move both forward and backward, as they try to 
match characters to a pattern.

Really, anything that uses an NFA or DFA to define patterns would 
benefit from fast bidirectional iteration...

--benji