Documentation for Unshaped

[femtomc]

It's mentioned a few times in the documentation that JAX can trace at higher levels of abstraction (using a lattice value called Unshaped).

Is there any documentation on this besides mention in a few parts of the docs?

Motivation: I'm considering ways to encode sum type - like Pytree instances, and I want to fully know the extent of the tracer, and what type values I can work with.