code: def __tokenize_segments(self):
          
          for seg_node_id in self.segments:
              segment_toks = self.node[seg_node_id][self.ns+':text'].split()
              for i, tok in enumerate(segment_toks):
                  tok_node_id = '{0}:{1}_{2}'.format(self.ns, seg_node_id, i)
                  self.add_node(tok_node_id, layers={self.ns, self.ns+':token'},
                                attr_dict={self.ns+':token': tok, 'label': tok})
                  self.tokens.append(tok_node_id)
                  self.add_edge(seg_node_id, tok_node_id,
                                layers={'rst', 'rst:token'},
                                edge_type=EdgeTypes.spanning_relation)
docs: tokenizes every RS3 segment (i.e. an RST nucleus or satellite).
          for each token, a node is added to the graph, as well as an edge from
          the segment node to the token node. the token node IDs are also added
          to ``self.tokens``.