code: def dt(self, start_node=None):
          
          if start_node is None:
              return self.root2tree(start_node=start_node)
  
          elem_id = start_node
          if elem_id not in self.elem_dict:
              return []
  
          elem = self.elem_dict[elem_id]
          elem_type = elem['element_type']
  
          assert elem_type in ('segment', 'group')
  
          if elem_type == 'segment':
              return self.segment2tree(
                  elem_id, elem, elem_type, start_node=start_node)
  
          else:
              return self.group2tree(
                  elem_id, elem, elem_type, start_node=start_node)
docs: main method to create an RSTTree from the output of get_rs3_data().
  
          TODO: add proper documentation