are distances to determine which vertex
  is closer to the source vertex.  The <tt>DistanceCompareFunction</tt> type
  must be a model of <a
  href="http://www.sgi.com/tech/stl/BinaryPredicate.html">Binary
  Predicate</a> and have argument types that match the value type of
  the <tt>DistanceMap</tt> property map.<br>

  <b>Default:</b>
  <tt>std::less&lt;D&gt;</tt> with <tt>D=typename
  property_traits&lt;DistanceMap&gt;::value_type</tt><br>
</blockquote>

IN: <tt>distance_combine(CombineFunction distance_