xparse documentation: star example has wrong order. · Issue #147 · latex3/latex3 · GitHub
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LaTeX宏参数技巧- 知乎
The xparse package Document command parser
expl3 - Custom arguments in an environment xparse - TeX - LaTeX Stack Exchange
expansion - Why does xparse manual talk people into thinking that reliable expandable checking for optional arguments is possible? - TeX - LaTeX Stack Exchange