bL construct <tt>DEFINE</tt>. See also the similar construct <tt>DEFPOLYMORPHIC</tt> which is used to define generic functions which switch off the type of the first argument.</p>
<p>Argument lists for user-defined function can only be of this form:</p>
<blockquote><pre>
<strong>(</strong> {var}* [<strong>&amp;optional</strong> {var | <strong>(</strong> var [initform [supplied-var]] <strong>)</strong> }*] <strong>)</strong>
</pre></blockquote>
<p>In short, SubL supports <tt>&amp;optional</tt> arguments