code: def validate(self, val):
          
          self.validate_type_only(val)
          if not hasattr(val, '_tag') or val._tag is None:
              raise ValidationError('no tag set')
          return val
docs: For a val to pass validation, it must have a _tag set. This assumes
          that the object validated that _tag is a valid tag, and that any
          associated value has also been validated.