[ben-schaaf]

> Can be flattened, to approximately halve the length of the program like this

No it can't. You're missing the recursive call to `eval`.

[nequo]

It looks like if let guards could help with flattening it in nightly but not in stable[1]:

https://rust-lang.github.io/rfcs/2294-if-let-guard.html

[1] https://github.com/rust-lang/rust/issues/51114