- [ ] Factorize input size checks (like Parsley's piggy bank).

- [ ] Golden tests using more complex grammars.

- [ ] Error messages also based upon: [A Parsing Machine for Parsing Expression Grammars with Labeled Failures](https://dl.acm.org/doi/10.1145/2851613.2851750)

- [ ] Consider introducing registers like in ParsleyHaskell.

- [ ] Concerning the unusual `pure :: H.Haskell a -> repr a`,
  it may be acceptable to use `H.Haskell` only internally.

- [ ] Move the `Symantic.Univariant.*` modules into a separate package, maybe `symantic-base`.