code: public boolean add(DirectedTypedEdge<T> e) {
          if (e.from() == rootVertex) 
              return add(outEdges, e.to(), e.edgeType());
          else if (e.to() == rootVertex) 
              return add(inEdges, e.from(), e.edgeType());
          return false;
      }
docs: Adds the edge to this set if one of the vertices is the root vertex and
  if the non-root vertex has a greater index that this vertex.