code: private EdgeSet<T> addIfAbsent(int v) {
          S edges = getEdgeSet(v);
          if (edges == null) {
              edges = createEdgeSet(v);
              vertexToEdges.put(v, edges);
          }
          return edges;
      }
docs: Returns the {@link EdgeSet} for this vertex, adding the vertex to this
  graph if abstent.