s an edge connecting vertex <tt class="docutils literal"><span class="pre">u</span></tt> to vertex <tt class="docutils literal"><span class="pre">v</span></tt> in graph
<tt class="docutils literal"><span class="pre">g</span></tt>. For bidirectional and undirected graphs, either vertex <tt class="docutils literal"><span class="pre">u</span></tt> or
vertex <tt class="docutils literal"><span class="pre">v</span></tt> must be local; for directed graphs, vertex <tt class="docutils literal"><span class="pre">u</s