heavily refactor everything

[leontrolski]

I'm not suggesting you merge this, just though you might find it interesting :-)

I really liked the core of your code, but thought I'd have a go at translating into my style, I'm probably going to use a modified version in a personal project if that's all cool?

Thanks for the great work!

[andychu]

This looks nice! The static types are a nice addition. I suggest just publishing it and then I can link to it for people who would like a different style.

Actually it relates precisely to my point here: http://www.oilshell.org/blog/2016/11/05.html

So I can link to your fork there. I don't think I will modify this code again so it makes sense to have two separate copies.