[mingodad]

It's not true "the best chance you have is reading the 14 thousand-line parse.y file and trying to understand it", there are several tools to navigate yacc/bnf style grammars like https://www.bottlecaps.de/convert/ and it's companion https://www.bottlecaps.de/convert/ that make relatively easy to understand/document/debug/compare the grammar.

Just added several ruby grammars here https://github.com/mingodad/plgh/tree/main/ruby they are converted (mainly using https://www.bottlecaps.de/convert/) to an EBNF understood by https://bottlecaps.de/rr/ui to generate navigable railroad diagrams.

Copy and paste the EBNF on https://www.bottlecaps.de/rr/ui on the tab "Edit Grammar" the click on the tab "View Diagram" to see/download a navigable railroad diagram.