function _deletePayment(uint64 paymentId) internal {
        uint listIndex = payments[paymentId].listIndex;
        paymentIds[listIndex] = paymentIds[paymentIds.length.sub(1)];
        payments[paymentIds[listIndex]].listIndex = listIndex;
        delete payments[paymentId];
        paymentIds.length = paymentIds.length.sub(1);
    }