rem: transaction -> voucher = NULL;
add: transaction -> voucher = "";
context: gboolean gsb_data_transaction_set_voucher ( gint no_transaction,					    const gchar *voucher ){    struct_transaction *transaction;    transaction = gsb_data_transaction_get_transaction_by_no ( no_transaction);    if ( !transaction )	return FALSE;    if ( voucher	 &&	 strlen (voucher))	transaction -> voucher = g_strdup (voucher);    else	transaction -> voucher = NULL;        return TRUE;}