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

def _add_token_node_ids_to_span_nodes(self):
        """
        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
        """
        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]