code: public Set<WeightedDirectedTypedEdge<T>> inEdges(int vertex) {
          SparseWeightedDirectedTypedEdgeSet<T> edges = vertexToEdges.get(vertex);
          return (edges == null) 
              ? Collections.<WeightedDirectedTypedEdge<T>>emptySet()
              : new EdgeListWrapper(edges.incoming());
      }
docs: {@inheritDoc}