code: def _add_sentence(self, sentence, ignore_traces=True):
          
          self.sentences.append(self._node_id)
          # add edge from document root to sentence root
          self.add_edge(self.root, self._node_id, edge_type=dg.EdgeTypes.dominance_relation)
          self._parse_sentencetree(sentence, ignore_traces=ignore_traces)
          self._node_id += 1
docs: add a sentence from the input document to the document graph.
  
          Parameters
          ----------
          sentence : nltk.tree.Tree
              a sentence represented by a Tree instance