code: def _add_token_node_ids_to_span_nodes(self):
          
          span_dict = defaultdict(list)
          for span_edge in self._spanning_relation_ids:
              token_node_id = self.edges[span_edge].target
              span_node_id = self.edges[span_edge].source
              span_dict[span_node_id].append(token_node_id)
  
          for span_node_id in span_dict:
              self.nodes[span_node_id].tokens = span_dict[span_node_id]
docs: Adds to every span node the list of tokens (token node IDs) that belong
          to it.
  
          SpanNode.tokens - a list of `int` ids of `TokenNode`s