code: def __add_sentence_root_node(self, sent_number):
          
          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
docs: 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