[Wilfred]

Thanks for the feedback. This is really hard to explain.

Autochrome has a brilliant worked example https://fazzone.github.io/autochrome.html and it still took me several readings before it clicked.

Every graph vertex represents a pair of pointers (or positions) to AST nodes. So in the example, the start program is `A` and the end program is `X A`. The positions point to AST nodes in these programs.

I try to use 'vertex' consistently for graph vertices, to avoid confusion with AST/s-expression nodes. If you have any suggestions for better terminology I'd be very interested too :)