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.