interface HarBankkonto {
    public void settInnIKonto(double sum);
    public void trekkFraKonto(double sum) throws IkkePengerException;
    public double hentSaldo();
    public void settSaldo(double sum);
}

interface KanKjope extends HarBankkonto {
    public void kjop(KanSelge selger, double pris);
}

interface KanSelge extends HarBankkonto {
    public void selg(double pris) throws IkkePengerException;
}