This needs a title

Highest priority:

• Clean up my 'parent vertex mapping' thingy, and come up with meaningful terms to discuss it.
• Terminology: "run" a rule versus "evaluate" a rule.
• Do transforms compose in the reverse of automata_scratch? This appears to be the case.

Important but less critical:

• Why must I repeat myself so much in these definitions?
• Consider trampolining to_mesh. My call stack seems needlessly deep in spots. Can I make tail-recursive?
• Grep for all TODOs in code, really.
• Look at everything in README.md in automata_scratch.
• Implement some of the tougher examples from the above too, e.g. the triple nested spiral. See examples.py.
• Actual Rust-style docs!

If I'm bored:

• Fix links in tri_mesh docs that use relative paths & do a PR?
• Look in https://www.nalgebra.org/quick_reference/# for "pour obtain". Can I fix this somehow? Looks like a French-ism that made its way in.