code: public Set<DirectedTypedEdge<T>> getEdges(int vertex1, int vertex2) {
          SparseDirectedTypedEdgeSet<T> edges = vertexToEdges.get(vertex1);
          return (edges == null) 
              ? Collections.<DirectedTypedEdge<T>>emptySet()
              : edges.getEdges(vertex2);
      }
docs: {@inheritDoc}