With https://mingodad.github.io/parsertl-playground/playground/ an Yacc/Lex compatible online editor/tester we can test/debug/develop grammars (select FSYacc parser from Examples then click Parse to see a parse tree for the content in Input source, also FSLex is there).
I hope this tool can help develop/debug/document Yacc/Lex like grammars.
Any feedback is welcome !