====== 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:


<code latex>
\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
</code>

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.


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//Source:// [[faquk>FAQ-subverttoks|Subverting a token register]]

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