### Subverting a token register

A common requirement is to “subvert” a token register that other macros may use. The requirement arises when you want to add something to a system token register (`\``output` or `\``every*`), but know that other macros use the token register, too. (A common requirement is to work on `\``everypar`, but LaTeX changes `\``everypar` at every touch and turn.)

The following technique, due to David Kastrup, does what you need, and allows an independent package to play the exact same game:

>     \let\mypkg@@everypar\everypar
>     \newtoks\mypkg@everypar
>     \mypkg@everypar\expandafter{\the\everypar}
>     \mypkg@@everypar{\mypkgs@ownstuff\the\mypkg@everypar}
>     \def\mypkgs@ownstuff{%
>       <stuff to do at the start of the token register>%
>     }
>     \let\everypar\mypkg@everypar

As you can see, the package (mypkg)

- creates an alias for the existing “system” `\``everypar` (which is frozen into any surrounding environment, which will carry on using the original);
- creates a token register to subvert `\``everypar` and initialises it with the current contents of `\``everypar`;
- sets the “old” `\``everypar` to execute its own extra code, as well as the contents of its own token register;
- defines the macro for the extra code; and
- points the token `\``everypar` at the new token register.

and away we go.

The form `\``mypkg@...` is (sort of) blessed for LaTeX package internal names, which is why this example uses macros of that form.

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=subverttoks