TeX source for the "category codes" post

This post gives the TeX source for a previous post on Category codes in TeX Save this text in a "hash-in-refs.tex" file and process it with the following command: tex hash-in-refs.tex

\input eplain
\enablehyperlinks

\newcount\exno
\exno=0
\newcount\answ
\answ=0

{
  \catcode`#=12
  \catcode`!=6
  \gdef\exercise{
    \global\advance\exno by 1 \href{#ex\the\exno}{Exercise \the\exno}
  }
  \gdef\exercisearg!1{
    \global\advance\exno by 1 \href{#ex\the\exno}{Exercise \the\exno} ({\it !1})
  }
}

\def\answer{
  \global\advance\answ by 1
  {\xrdef{ex\the\answ} Answer of exercise \the\answ}
}

\exercise
This is the first exercise.
\medskip

\exercise
This is the second exercise.
\medskip

\exercisearg{Category codes\dots}
\par
{
An exercise on category codes: how to use '/' for commands instead of
\catcode`/=0
\catcode`\\=12
'{/tt \}' ?
% '$/backslash$'?
}

(For instance, {\tt /def/a$\{$macro a$\}$}).


\vfill

\eject %page break

\answer
Answer to the first exercise.

\eject

\answer
Answer to the second exercise.

\eject

\answer

The ``escape'' character '$\backslash$' is of category 0. Hence,
{
 \catcode`/=0
 \catcode`\\=12
 /tt \catcode/catcode`//=12 `/=0
} does the trick.
 
\vfill

\end