# What is “\C” with xelatex and beamer?

When compiling a beamer document using xelatex, I encountered a problem with the macro \C. Basically, I want to use that to stand for the set of complex numbers so I want to redefine it to be \mathbb{C}, but xelatex complains that it is already defined. Using \show, I can see that it is defined to be:

> \C=macro:
->\PU-cmd \C \PU\C .


but a texmf-wide grep doesn't show me anything sensible on what that actually means! So I don't know if it's safe to clobber it or not.

Here's the minimal test case. Note that the issue only shows up with xelatex, not with pdflatex or lualatex.

\documentclass{beamer}

\begin{document}
\show\C
\end{document}


It feels like something font-y, since if I remove the \show (i.e., actually try to use the command), then I get the error:

! LaTeX Error: Command \C unavailable in encoding OT1.

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