adds the root node of a sentence to the graph and the list of sentences
        (``self.sentences``). the node has a ``tokens` attribute, which
        contains a list of the tokens (token node IDs) of this sentence.

        Parameters
        ----------
        sent_number : int
            the index of the sentence within the document

        Results
        -------
        sent_id : str
            the ID of the sentence

def __add_sentence_root_node(self, sent_number):
        """
        adds the root node of a sentence to the graph and the list of sentences
        (``self.sentences``). the node has a ``tokens` attribute, which
        contains a list of the tokens (token node IDs) of this sentence.

        Parameters
        ----------
        sent_number : int
            the index of the sentence within the document

        Results
        -------
        sent_id : str
            the ID of the sentence
        """
        sent_id = 's{}'.format(sent_number)
        self.add_node(sent_id, layers={self.ns, self.ns+':sentence'},
                      tokens=[])
        self.add_edge(self.root, sent_id,
                      layers={self.ns, self.ns+':sentence'},
                      edge_type=EdgeTypes.dominance_relation)
        self.sentences.append(sent_id)
        return sent_id