code: def is_closed(self):
          
          for t in self.smi_vector:
              found = False
              for s in self.sm_vector:
                  if self.observation_table[s] == self.observation_table[t]:
                      self.equiv_classes[t] = s
                      found = True
                      break
              if not found:
                  return False, t
          return True, None
docs: _check if the observation table is closed.
          Args:
              None
          Returns:
              tuple (bool, str): True if the observation table is
              closed and false otherwise. If the table is not closed
              the escaping string is returned.