docs: def add_state_validator(self, validator):
          """
          Add entity state validator
          :param validator: a validator, implementing AbstractValidator
          :return: None
          """
          if not isinstance(validator, AbstractValidator):
              err = '{} is not a subclass of {}'
              raise InvalidValidator(err.format(validator, AbstractValidator))
  
          if validator not in self.state:
              self.state.append(validator)