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<title>UK TeX FAQ -- question label protect</title>
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<h3>What’s the reason for ‘protection’?</h3>
<p>Sometimes LaTeX saves data it will reread later. These data are
often the argument of some command; they are the so-called moving
arguments. (‘Moving’ because data are moved around.) Places to look for
are all arguments that may go into table of contents, list of figures,
<em>etc</em>.; namely, data that are written to an auxiliary file and
read in later. Other places are those data that might appear in head-
or footlines. Section headings and figure captions are the most
prominent examples; there’s a complete list in Lamport’s book
(see <a href="FAQ-books.html">TeX-related books</a>).
<p>
<p>What’s going on really, behind the scenes? The commands in the moving
arguments are already expanded to their internal structure during the
process of saving. Sometimes this expansion results in invalid TeX
code when processed again. “<code>\</code><code>protect</code><code>\</code><code>cmd</code>” tells LaTeX to save
<code>\</code><code>cmd</code> as <code>\</code><code>cmd</code>, without expansion.
<p>What is a ‘fragile command’? It’s a command that expands into illegal
TeX code during the save process.
<p>What is a ‘robust command’? It’s a command that expands into legal
TeX code during the save process.
<p>Again, commands are marked as ‘robust’ or ‘fragile’, as they’re
defined in Lamport’s book. Sadly, some commands are robust in
LaTeX itself, but are redefined by some packages to be fragile; the
<code>\</code><code>cite</code> command commonly suffers this treatment.
<p>No-one (of course) likes this situation; the LaTeX3 team have
removed the need for protection of some things in the production of
LaTeX2e, but the techniques available to them within current
LaTeX mean that this is an expensive exercise. It remains a
long-term aim of the team to remove all need for these things.
<p><p>This question on the Web: <a href="http://www.tex.ac.uk/cgi-bin/texfaq2html?label=protect">http://www.tex.ac.uk/cgi-bin/texfaq2html?label=protect</a>
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