code: public boolean add(Edge e) {
          int toAdd = -1;
          if (e.from() == rootVertex) 
              toAdd = e.to();        
          else if (e.to() == rootVertex)
              toAdd = e.from();
          else
              return false;
          boolean b = edges.add(toAdd);
          return b;
      }
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.