docs: def annotated(self):
          """
          Return an AnnotatedGraph with the same structure
          as this graph.
  
          """
          annotated_vertices = {
              vertex: AnnotatedVertex(
                  id=vertex_id,
                  annotation=six.text_type(vertex),
              )
              for vertex_id, vertex in zip(itertools.count(), self.vertices)
          }
  
          annotated_edges = [
              AnnotatedEdge(
                  id=edge_id,
                  annotation=six.text_type(edge),
                  head=annotated_vertices[self.head(edge)].id,
                  tail=annotated_vertices[self.tail(edge)].id,
              )
              for edge_id, edge in zip(itertools.count(), self.edges)
          ]
  
          return AnnotatedGraph(
              vertices=annotated_vertices.values(),
              edges=annotated_edges,
          )