code: def _add_span_node_ids_to_token_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[token_node_id].append(span_node_id)
  
          for token_node_id in span_dict:
              self.nodes[token_node_id].spans = span_dict[token_node_id]
docs: Adds to every token node the list of spans (span node IDs) that it
          belongs to.
  
          TokenNode.spans - a list of `int` ids of `SpanNode`s