1. What cost the most: finding where the error comes from. The brief guessed it was a parser message. It is a rustc message on the lowered Rust, shown with a capital letter, which is why no grep of crates/ found it. One nikaia lower of the probe showed let mut (at, depth) and for (mut a, mut b) at once. After that I spent about 6 turns finding the per-name mut logic and the for emit sites in emit_stmt.nika, and checking whether the checker keeps a set of assigned names (it doesn’t).

2. What the brief could have handed me:
   • “Run nikaia lower on the probe first.” The Locations section sent me towards the parser and the grammar, which I never needed.
   • That warnings like “Variable does not need to be mutable.” are rustc lints passed through, so there is no Nikaia unused_mut to find.
   • That emit_stmt_let (tuple branch) and the for emit in emit_stmt.nika are where mut gets written.
3. Checks run:
   • Useful: probes on the old and new binaries, the scratch-project test with both binaries (it failed before the fix, passes after), the it grep for the old lowering text, cargo fmt, pre-push-check.sh.
   • Possibly wasted: the second cargo build “for the fixpoint” did nothing (0.1 s). Full clippy --all-targets for one lowered file was probably more than needed, though the rules ask for it.
   • Gaps in my report:
     • I wrote Gate: green from the cheap checks only.
     • I wrote Pushed: none myself; the commit script was supposed to write that line.
     • The commit subject has no (Fixes #1126). The Fixes: field puts it in the message instead.
     • The disk figures are not in the report file.
4. One change: add to builder.md’s traps: “A refusal or warning you can’t find in crates/ is probably rustc’s on the lowered Rust: nikaia lower the probe and read the .rs before looking for the message.” In briefs, the architect should do that lower before writing Locations.

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