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Highest priority:

• See ram_horn_* and curve_horn_* TODOs: these are both solved by some sort of parent-vertex-mapping layer. The only other way around this that I need is to require that rule functions exist in explictly separate forms.
• Consider trampolining to_mesh. My call stack seems needlessly deep in spots. Can I make tail-recursive?

Important but less critical:

• 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.