private boolean enteredViaVertex0( int anEdgeIndex)
   {
      if (anEdgeIndex < 1 || anEdgeIndex > 5)
         throw new IllegalArgumentException( "Edge index must be in range [1..5], was " + anEdgeIndex);
      else
      {
         boolean retval;
         Edge cur = myEdge[anEdgeIndex];
         Edge prev = myEdge[anEdgeIndex-1];
         retval = (cur == prev.getEdge1Plus() && cur.getVertex0().equals( prev.getVertex1()))
               || (cur == prev.getEdge0Plus() && cur.getVertex0().equals( prev.getVertex0()));
         return retval;
      }      
   }