code: def get_position(self, rst_tree, node_id=None):
          
          if node_id is None:
              node_id = self.root_id
  
          if node_id in rst_tree.edu_set:
              return rst_tree.edus.index(node_id)
  
          return min(self.get_position(rst_tree, child_node_id)
                     for child_node_id in rst_tree.child_dict[node_id])
docs: Get the linear position of an element of this DGParentedTree in an RSTTree.
  
          If ``node_id`` is given, this will return the position of the subtree
          with that node ID. Otherwise, the position of the root of this
          DGParentedTree in the given RSTTree is returned.