lable with two parameters - the descriptor of
vertex and the graph instance - and returning bool. The call is made
immediately after call to 'discover_vertex' visitor
event. If <tt>true</tt> is returned, the out edges of the vertex are
not examined, as if they don't exist.<br>

  <b>Python</b>: Unsupported parameter.
</blockquote>
<P>

<h3>Complexity</h3>

Time complexity is <i>O(E)</i>.


<h3>Notes</h3>

<p><a name="1">[1]</a> 
  Since the visitor parameter is passed by value, if your visitor
  contains st