public function setAlternativeAccount($iban1, $swift1 = null, $iban2 = null, $swift2 = null)
    {
        if ($swift1 !== null) {
            $iban1 .= '+' . $swift1;
        }
        if ($iban2 !== null) {
            if ($swift2 !== null) {
                $iban2 .= '+' . $swift2;
            }
            $iban1 .= ',' . $iban2;
        }

        return $this->add('ALT-ACC', $iban1);
    }