code: def _validate_class(self, cl):
          
          if cl not in self.schema_def.attributes_by_class:
              search_string = self._build_search_string(cl)
              err = self.err(
                  "{0} - invalid class", self._field_name_from_uri(cl),
                  search_string=search_string)
              return ValidationWarning(ValidationResult.ERROR, err['err'],
                                       err['line'], err['num'])
docs: return error if class `cl` is not found in the ontology