code: def _flag_node_as_del_and_del_in_by_target(self, node_id):
          
          self.nodes_deleted.append(node_id)
          etp = self._edge_by_target.get(node_id)
          if etp is not None:
              del self._edge_by_target[node_id]
docs: Flags a node as deleted, and removes it from the _edge_by_target (and parent's edge_by_source), if it is still found there.
          Does NOT remove the node's entries from self._edge_by_source.