code: def add_token_layer(self, words_file, connected):
          
          for word in etree.parse(words_file).iterfind('//word'):
              token_node_id = word.attrib['id']
              self.tokens.append(token_node_id)
              token_str = ensure_unicode(word.text)
              self.add_node(token_node_id,
                            layers={self.ns, self.ns+':token'},
                            attr_dict={self.ns+':token': token_str,
                                       'label': token_str})
              if connected:
                  self.add_edge(self.root, token_node_id,
                                layers={self.ns, self.ns+':token'})
docs: parses a _words.xml file, adds every token to the document graph
          and adds an edge from the MMAX root node to it.
  
          Parameters
          ----------
          connected : bool
              Make the graph connected, i.e. add an edge from root to each
              token.