code: def __add_sentence_to_document(self, sentence):
          
          sentence_graph = TigerSentenceGraph(sentence)
          self.tokens.extend(sentence_graph.tokens)
          sentence_root_node_id = sentence_graph.root
  
          self.add_nodes_from(sentence_graph.nodes(data=True))
          self.add_edges_from(sentence_graph.edges(data=True))
          self.add_edge(self.root, sentence_root_node_id,
                        layers={self.ns, self.ns+':sentence'},
                        edge_type=EdgeTypes.dominance_relation)
          self.sentences.append(sentence_root_node_id)
docs: Converts a sentence into a TigerSentenceGraph and adds all
          its nodes, edges (and their features) to this document graph.
  
          This also adds a ``dominance_relation`` edge from the root node of this
          document graph to the root node of the sentence and appends the
          sentence root node ID to ``self.sentences``.
  
          Parameters
          ----------
          sentence : lxml.etree._Element
              a sentence from a TigerXML file in etree element format