           DistancePropertyMap distance)
  {
    typedef typename graph_traits&lt;Graph&gt;::vertex_descriptor Vertex;
    Vertex u = source(e,g), v = target(e,g);
    if ( get(distance, u) + get(weight, e) &lt; get(distance, v)) {
      put(distance, v, get(distance, u) + get(weight, e));
      return true;
    } else
      return false;
  }
</PRE>

The function <TT>get()</TT> returns a copy of the property value.  There
is a third function in the property map interface, <TT>at()</TT>,
that returns a refer