code: def _add_token_to_document(self, token_string, token_attrs=None):
          
          token_feat = {self.ns+':token': token_string}
          if token_attrs:
              token_attrs.update(token_feat)
          else:
              token_attrs = token_feat
          token_id = 'token_{}'.format(self.token_count)
          self.add_node(token_id, layers={self.ns, self.ns+':token'},
                        attr_dict=token_attrs)
          self.token_count += 1
          self.tokens.append(token_id)
          return token_id
docs: add a token node to this document graph