1. What cost the most. Three lower-std round trips, each over a refusal about borrowing that I could have avoided by writing the code more plainly first:
   • for bound in kept pushed a ref CheckLocal into a Vec.
   • let below = match … => pat.node was refused with NK2106.
   • A helper returning args[0] (or args[0].node) as ref Spanned[Pattern] was refused with NK1104, but returning a match binding works. I worked around it by writing two arms. This may be a defect, and my report should have listed it under Found: with a probe; I left it out.

   After those, the gate ran once, long but in one call.

2. What I looked up that the brief could have handed me:
   • bindings_of has one caller (check_function.nika:324).
   • check_defaults_folds_in does not depend on the action’s frame, so fold steps were unaffected.
   • text_tiers.nika bind_grammar_pattern already over-binds and needs no change.
   • The action prelude’s line in emit_grammar.nika (~408).
   • Span holds start: u32.
   • How to get a .stderr: nikaia check inside tests/compiler checks the whole project and showed another test’s error. I needed a scratch project with tests/NAME.nika.
3. Checks I ran:
   • Useful: probes a–e on the unfixed binary, then after the build, plus one extra Bytes probe; packages/yaml tests; the gate, whose test step covered the whole tests/compiler package at both settings.
   • Partly wasted: the first nikaia check for the .stderr; the grep for text( uses across examples (no hits in grammars); the gate --dry-run to confirm the test step’s scope. That scope is in builder.md already.
4. One change that would have saved the most. In builder-facts, Traps, a note on writing tools: “a function cannot hand back xs[i] as ref; a let cannot keep a field of a borrowed value; iterating a local Vec yields ref. Write the loop inside each match arm.” Add the .stderr recipe too: a scratch dir with tests/NAME.nika, then nikaia check tests/NAME.nika.

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