code: def edge_by_target(self):
          
          if self._edge_by_target is None:
              self._edge_by_target = reverse_edge_by_source_dict(self._edge_by_source_id,
                                                                 self._nexson_tree['^ot:rootNodeId'])
          return self._edge_by_target
docs: Returns a reference to the dict of target node id to (edge_id, edge)