span></tt> and <tt class="docutils literal"><span class="pre">v</span></tt> can be either vertex descriptors or, if
the graph uses named vertices, the names of vertices. If no vertex
with the given name exists, the internal vertex constructor will be
invoked to create a new vertex property and a vertex with that
property will be added to the graph implicitly. The default internal
vertex constructor throws an exception.</p>
<hr class="docutils" />
<pre class="literal-block">
unspecified add_edge(vertex_descr