private function validateInput($haystack, $keys)
    {
        if (!is_array($haystack) || empty($haystack)) {
            throw new \InvalidArgumentException(
                "Provided input array is either empty or not an array."
            );
        }
        if (!is_array($keys) || empty($keys)) {
            throw new \InvalidArgumentException(
                "Provided input keys argument is either empty or not an array."
            );
        }
    }