Post-audit downstream-LLM-author field test of the post-removal
Form-A surface. 4 fresh real-world programs written from DESIGN.md +
form_a.md + public examples only (no compiler source): running
sum-of-squares accumulator (loop/recur → 385), grade cascade
(let-threaded if → 4/2/0), Horner polynomial straight-line build-up
(let-shadow chain → 73), multi-state bracket scanner threading
(rest,depth,ok) by tail recursion → true/false. 0 bugs, 0 friction,
0 spec_gap, 4 working — every task clean and correct on the first
try with only let/if/loop/recur; all parse|render|parse
byte-identical; zero mut/var/assign tokens produced (a doc-faithful
author did not reach for the removed construct).
The closest-to-friction case (multi-state machine restates the full
state tuple per tail-call) is correctly classified working, not a
spec_gap: that explicit-dataflow cost is the local-reasoning pillar
working as designed; mut's implicit cross-iteration persistence is
exactly the failure class the removal eliminates. mut-keyword
rejection is fail-closed with a diagnostic that enumerates the
surviving forms (no tombstone, no-nostalgia, as spec'd).
Removal thesis empirically CONFIRMED — mut was redundant with
let/if/loop/recur in 100% of the fresh tasks. The
remove-mut-var-assign milestone is fully ratified and CLOSED:
spec+plan+iter, audit clean (architect [high] form_a.md fixed in
.tidy, bench causally exonerated), fieldtest clean. Roadmap P0
flipped [~]→[x]; WhatsNew user-facing entry appended.
Brummel
5bb721178f