are given first—nested in a <tt class="docutils literal">(required … )</tt>
clause—as a series of two-element tuples describing each parameter
name and any requirements on the argument type.  In this case there
is only a single required parameter, so there's just a single
tuple:</p>
<pre class="compound-middle literal-block">
(required <strong>(graph, *)</strong> )
</pre>
<p class="compound-last">Since <tt class="docutils literal">depth_first_search</tt> doesn't require any particular type
for its <tt