axed(e, g)</tt></b>
  is invoked on edge <i>(u,v)</i> if <i>d[u] + w(u,v) < d[v]</i>.
  The edge <i>(u,v)</i> that participated in the last
  relaxation for vertex <i>v</i> is an edge in the shortest paths tree. 
<li><b><tt>vis.discover_vertex(v, g)</tt></b>
  is invoked on vertex <i>v</i> when the edge 
  <i>(u,v)</i> is examined and <i>v</i> is WHITE. Since
  a vertex is colored GRAY when it is discovered,
  each reacable vertex is discovered exactly once.
<li><b><tt>vis.edge_not_relaxed(e, g)</tt></b>
  