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<h3 class="section">16.3 Optional and Repeated Arguments</h3>
<p><a name="index-Optional-and-repeated-arguments-947"></a><a name="index-Repeated-and-optional-arguments-948"></a><a name="index-Arguments_002c-repeated-and-optional-949"></a><a name="index-Syntax_002c-optional-_0026-repeated-arguments-950"></a><a name="index-Meta_002dsyntactic-chars-for-arguments-951"></a>
Some entities take optional or repeated arguments, which may be
specified by a distinctive glyph that uses square brackets and
ellipses. For example<!-- /@w -->, a special form often breaks its argument list
into separate arguments in more complicated ways than a
straightforward function.
<!-- This is consistent with Emacs Lisp Reference manual -->
<p>An argument enclosed within square brackets is optional.
Thus, [<var>optional-arg</var>] means that <var>optional-arg</var> is optional.
An argument followed by an ellipsis is optional
and may be repeated more than once.
<!-- This is consistent with Emacs Lisp Reference manual -->
Thus, <var>repeated-args</var>‘<samp><span class="samp">...</span></samp>’ stands for zero or more
arguments. Parentheses are used when several arguments are grouped
into additional levels of list structure in Lisp.
<p>Here is the <code>@defspec</code> line of an example of an imaginary
special form:
<blockquote>
<div class="defun">
— Special Form: <b>foobar</b> (<var>var </var>[<var>from to </var>[<var>inc</var>]])<var> body<small class="dots">...</small><a name="index-foobar-952"></a></var><br>
</div>
</blockquote>
<p class="noindent">In this example, the arguments <var>from</var> and <var>to</var> are optional,
but must both be present or both absent. If they are present,
<var>inc</var> may optionally be specified as well. These arguments are
grouped with the argument <var>var</var> into a list, to distinguish them
from <var>body</var>, which includes all remaining elements of the
form.
<p>In a Texinfo source file, this <code>@defspec</code> line is written like
this (except it would not be split over two lines, as it is in this
example).
<pre class="example"> @defspec foobar (@var{var} [@var{from} @var{to}
[@var{inc}]]) @var{body}@dots{}
</pre>
<p class="noindent">The function is listed in the Command and Variable Index under
‘<samp><span class="samp">foobar</span></samp>’.
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