Adaptive subdivision - which means having to generalize past some vmap stuff.
Try some non-deterministic examples
Get identical or near-identical meshes to ramhorn_branch from Python. (Should just be a matter of tweaking parameters.)
See automata_scratch/examples.py and implement some of the tougher examples.
- twisty_torus, spiral_nested_2, & spiral_nested_3 are all that remain. To do them, I need to compose transformations (not in the matrix sense), but I also probably need to produce RuleEvals which always have xf of identity transformation since the Python code does not 'inherit' transforms unless I tell it to.

Important but less critical:

Look at performance.
- Start at to_mesh_iter(). The cost of small appends/connects seems to be killing performance.
- connect() is a big performance hot-spot: 85% of total time in one test, around 51% in extend(), 33% in clone(). It seems like I should be able to share geometry with the Rc (like noted above), defer copying until actually needed, and pre-allocate the vector to its size (which should be easy to compute).
Elegance & succinctness:
- Clean up ramhorn_branch because it's ugly.
- What patterns can I factor out? I do some things regularly, like: the clockwise boundaries, the zigzag connections.
- Declarative macro to shorten this Tag::Parent, Tag::Body nonsense - and perhaps force to groups of 3? Does this have any value, though, over just making helper functions like p(...) and b(...)?
- I'm near certain a declarative macros can simplify some bigger things like my patterns with closures (e.g. the Y combinator like method for recursive calls).
Docs on modules
Compute global scale factor, and perhaps pass it to a rule (to eventually be used for, perhaps, adaptive subdivision)
swept-isocontour stuff from /mnt/dev/graphics_misc/isosurfaces_2018_2019/spiral*.py
Catch-alls:
- Grep for all TODOs in code, really.
- Look at everything in README.md in automata_scratch.

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.
Multithread! This looks very task-parallel anywhere that I branch.
Would being able to name a rule node (perhaps conditionally under some compile-time flag) help for debugging?
Use an actual logging framework.