Refactor the formula parser to assume grammar invariants, replacing
Result-based error handling in tree walkers with infallible functions and
.expect(GRAMMAR_INVARIANT) calls.
This simplification is based on the guarantee that the Pest grammar already
validates the input structure. To verify these invariants and ensure
correctness, extensive new unit tests and property-based tests using
proptest have been added.
Co-Authored-By: fiddlerwoaroof/git-smart-commit (gemma-4-31B-it-UD-Q4_K_XL.gguf)
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