code: def _reorder_fields(self, ordering):
          
          if 'captcha' not in ordering:
              raise ImproperlyConfigured(
                  "When using 'FLUENT_COMMENTS_FIELD_ORDER', "
                  "make sure the 'captcha' field included too to use '{}' form. ".format(
                      self.__class__.__name__
                  )
              )
          super(CaptchaFormMixin, self)._reorder_fields(ordering)
  
          # Avoid making captcha required for previews.
          if self.is_preview:
              self.fields.pop('captcha')
docs: Test that the 'captcha' field is really present.
          This could be broken by a bad FLUENT_COMMENTS_FIELD_ORDER configuration.