public function never(array $keys) : Assoc
    {
        return $this->pipe(function (array $values) use ($keys) : Result {
            $result = Result::success();

            foreach ($keys as $key) {
                if (!isset($values[$key])) {
                    continue;
                }

                $result = $result->concat(
                    Result::failure("contains '$key'")
                );
            }

            return $result;
        });
    }