parse: expand \def macros in the mouth, instead of dropping the formula

[tannevaled]

What

A \def written inside a formula — a paper's own inline shorthand reaching the math layer (\def\R{\mathbb{R}}\R, \def\abs#1{\lvert#1\rvert}\abs{x}) — was an unknown command that dropped the whole equation. Macros defined in the preamble are already substituted upstream by go-tex/engine's math retry; the ones defined in the formula were not, and \def is a top math-drop cause across the arXiv corpus.

How

A TeX "mouth" pass (macro.go) runs before the parser (expandMacros, after resolveConditionals): it records each \def and expands every use — arguments spliced into #1…#n of the body, the result re-processed so nested and chained macros resolve — with a budget so a self- or mutually-recursive macro (\def\x{\x}) can't spin. It supports undelimited parameters (the inline form that occurs); a \def it cannot read (delimited parameters, missing/unbalanced body) or a use with too few arguments is left verbatim for the parser to report, never guessed at. Transparent to a def-free stream.

Verification

New macro_test.go: parameterless/parameterised macros, chained (\a→\b→x+y) and nested macros, expansion inside \frac arguments, single-token vs {group} args, an undeclared-#n body, and every rejecting path (bad name, delimited params, missing/unbalanced body, too few args, unbalanced arg, self-recursion halts). Full suite + -race + go vet + gofmt clean at 100% coverage.

Continues the worst-first math-drop robustness pass (after \char, #9). A follow-up bumps the engine's go-tex/math dependency so the corpus recovers end-to-end.

🤖 Generated with Claude Code