code: public LabeledBiDiNavigator<Vertex,Label> getLabeledBiDiNavigator() {
  	if(CACHING && lnav == null) {
  	    return lnav;
  	}
  	LabeledBiDiNavigator<Vertex,Label> lnav = _getLabeledBiDiNavigator();
  	if(CACHING) {
  	    this.lnav = lnav;
  	}
  	return lnav;
      }
docs: Returns a bi-directional labeled navigator through
  <code>LabeledDiGraph</code>.  The default implementation uses the
  forward navigator returned by <code>LabeledForwardNavigator</code>
  to traverse the entire graph and construct the list of
  backward arcs.
  
  <b>Note:</b> You MUST override at least one of
  <code>getLabeledBiDiNavigator</code> and
  <code>getLabeledForwardNavigator</code>.