In LaTeX code, `@` should be treated as a letter. There are any number of examples on tex.stackexchange, but *https://tex.stackexchange.com/questions/505192/undefined-control-sequence-on-beamerleftmargin-indentation/505348#505348* ```latex \begin{frame}[fragile]{E} \makeatletter \hskip-\beamer@leftmargin \makeatother \lipsum[2] \end{frame} ``` `\beamer@leftmargin` is a single token, but `beamer` is coloured and `@leftmargin` is left as unstyled text which makes the code very hard to read. Technically, @ is not always a letter, but it is almost always a letter when appearing in code sections and is a far better default in a syntax highlighter.