docs: def validate(self, value):
          """Validate field value."""
          if value is not None:
              if not isinstance(value, list):
                  raise ValidationError("field must be a list")
  
              for index, element in enumerate(value):
                  try:
                      self.inner.validate(element)
                  except ValidationError as error:
                      raise ValidationError("invalid element {}: {}".format(
                          index,
                          error.args[0],
                      ))
  
          super().validate(value)