code: def _add_token(self, token, parent_node='root'):
          
          if parent_node == 'root':
              parent_node = self.root
  
          token_node_id = 'token:{}'.format(self.token_count)
          self.add_node(token_node_id, layers={self.ns, self.ns+':token'},
                        attr_dict={self.ns+':token': token})
          self.add_edge(parent_node, token_node_id,
                        layers={self.ns},
                        edge_type=EdgeTypes.spanning_relation)
          self.tokens.append(token_node_id)
          self.token_count += 1
docs: add a token to this docgraph