By the way, the current doc is wrong about \char_set_catcode:nn.  For
example:

\char_set_catcode:nn {\@} {11} % Wrong
\char_set_catcode:nn {@} {11} % Wrong
\char_set_catcode:nn {`\@} {11} % Correct

So you need two numbers (not that surprising).
-- 
Joseph Wright