proximation algorithm.


<h3>Notes</h3>

<p><a name="1">[1]</a>
  Since the visitor parameter is passed by value, if your visitor
  contains state then any changes to the state during the algorithm
  will be made to a copy of the visitor object, not the visitor object
  passed in. Therefore you may want the visitor to hold this state by
  pointer or reference.
</p>
<p><a name="2">[2]</a>
  Passing an <tt>adjacency_list</tt> with a vertex <em>not</em> set selected by
  <tt>vecS</tt> will result in <i>O(n<sup