Tested McCarthy's head[a; b] notation (user request). Explicit structure
(brackets + semicolons) in the function-call shape LLMs know cold. Result
4-5/8 (model variance over two runs): below plain parens, above bracket-free.
Two watch-out catches kept it honest: (1) a trailing ';' before ']' — a normal
trailing separator Qwen wrote — was rejected by my too-strict parser; fixed to
tolerate it, round-trip preserved (without this M-expr looked unfairly bad).
(2) L4 then failed with 43 '[' vs 42 ']', a genuine Qwen bracket imbalance (not
truncation), the same depth-tracking failure as plain parens.
Why worse than plain parens despite being explicit: swapping () for [] does
nothing for the balance burden, and the semicolons ADD a second consistency
requirement Qwen does not reliably meet.
This sharpens the overall finding across five formats (annotated-parens 7/8 >
plain parens 6/8 > M-expr 4-5/8 > indent 3/8 > YAML 2/8): the discriminator is
not "explicit vs implicit" but REDUNDANT CUE vs EXTRA BURDEN. Depth-annotated
parens carry nesting redundantly so the model can't lose its place (best);
indentation moves tracking to an implicit channel (worst); M-expr semicolons
add bookkeeping on top of bracket balance (middle). Lever for an LLM-authored
language: redundant self-checking structure, not a lighter or more familiar
surface that carries extra bookkeeping. Synthesis in format-findings.md.
