public function rpoplpush(string $sourceList, string $destinationList)
    {
        // RPOP (get last value of the $sourceList)
        $rpopValue = $this->rpop($sourceList);

        // LPUSH (send the value at the end of the $destinationList)
        if (null !== $rpopValue){
            $this->lpush($destinationList, $rpopValue);
        }

        // return the rpop value;
        return $rpopValue;
    }