code: def is_valid(self, tree):
          
          conano_plaintext = etree.tostring(tree, encoding='utf8', method='text')
          token_str_list = conano_plaintext.split()
          for i, plain_token in enumerate(token_str_list):
              graph_token = self.node[self.tokens[i]][self.ns+':token']
              if ensure_unicode(plain_token) != graph_token:
                  sys.stderr.write(
                      "Conano tokenizations don't match: {0} vs. {1} "
                      "({2})".format(plain_token, graph_token))
                  return False
          return True
docs: returns true, iff the order of the tokens in the graph are the
          same as in the Conano file (converted to plain text).