 <tt class="docutils literal"><span class="pre">false</span></tt>. Also, if u and v are descriptors for
the same vertex (creating a self loop) and the graph is undirected,
then the edge will not be added and the flag will be <tt class="docutils literal"><span class="pre">false</span></tt>. When
the flag is <tt class="docutils literal"><span class="pre">false</span></tt>, the returned edge descriptor points to the
already existing edge.</p>
<p>The parameters <tt class="docutils literal"><span class="pre">u</