What does "\c!" means?

[Wei-Wei Guo <wwguocn@gmail.com>]

Dear all,

I'm reading the source file of t-bib. I find there are lots of '\c!', '\s!', and
'\v!'. What do they mean? I searched in wiki, TeXBook, ConTeXt manual, TeX impatient,
and mail list. Those commands appear some times, but no explanation.

Another question is related the following codes from Taco (thank you, Taco). There
are tokens like '\??pb @lang@`. What does it mean? Could you tell me, to get deep
understanding, what to read or which part to read in TeXBook or some other manuals.
I don't know what to search to find related topics.

>   \unprotect
>   \def\lang#1%
>    {\def\biblanguage{#1}%
>     \ifcsname \??pb @lang@#1\endcsname
>         \expanded{\mainlanguage[\getvalue{\??pb @lang@#1}]}%
>         \expanded{\language[\getvalue{\??pb @lang@#1}]}%
>      \fi \ignorespaces}
>    \protect
> 

Best wishes,
Wei-Wei
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