public function delete(/*# string */ $key)/*# : bool */
    {
        $ends = [ $this->front, $this->back ];
        foreach ($ends as $end) {
            if (!$end->delete($key)) {
                return $this->falseAndSetError(
                    $end->getError(),
                    $end->getErrorCode()
                );
            }
        }
        return $this->trueAndFlushError();
    }