add a sentence from the input document to the document graph.

        Parameters
        ----------
        sentence : nltk.tree.Tree
            a sentence represented by a Tree instance

def _add_sentence(self, sentence, ignore_traces=True):
        """
        add a sentence from the input document to the document graph.

        Parameters
        ----------
        sentence : nltk.tree.Tree
            a sentence represented by a Tree instance
        """
        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