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context/code/formula-and-io.md already says a new grammar production needs generator coverage or the proptests quietly stop describing the format. `kind_line` is an alternative of `category_entry`, so the grammar walk reaches it with no generator change — but that is precisely the claim the section exists to say must be checked rather than assumed. Probed over 500 files from generator::improv_file(): 14 carried a `kind: label` line and all 14 parsed to a CategoryKind::Label, so each 256-case proptest run exercises the production about seven times. The probe is not kept. Its rate depends on proptest's internal RNG, so as a standing assertion it would fail on a dependency bump for a reason that has nothing to do with the format — the measurement belongs in the argument, where it can be re-run and corrected. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH