code: public long numArcs() {
  	if(CACHING && (cachedNumArcs != -1)) {
  	    return cachedNumArcs;
  	}
  	long numArcs = 0;
  	for(Vertex v : vertices()) {
  	    numArcs += getForwardNavigator().next(v).size();
  	}
  	if(CACHING) {
  	    cachedNumArcs = numArcs;
  	}
  	return numArcs;
      }
docs: Returns the number of arcs in <code>this</code> directed
  graph.
  
  Complexity: linear in the size of the graph.  Cached if the
  <code>CACHING</code> argument to the constructor was true.