[texhax] implementation of x' in math

[Karl Berry]

Hi Alexandru,

    >> How is the translation of $x'$ into $x^{\prime}$ implemented?

See plain.tex and the TeXbook and/or TeX for the Impatient (I think we
talked about this in TFTI, although I don't recall for sure).

The short answer is:
\mathcode`\'="8000
This makes ' be an active character in math mode.

The definition starts like this:
{\catcode\'=\active \gdef'{^\bgroup\prim at s}}

\prim at s uses \futurelet to look for multiple consecutive ' characters.

Hope this helps,
karl