so test for 'payment' being an instance of 'IZUGFeRDTradeSettlementDebit' as 'IZUGFeRDTradeSettlement' and 'IZUGFeRDTradeSettlementPayment' return 'null' for 'getPaymentXML()' which is not allowed to be written to the XML file.
so test for 'payment' being an instance of 'IZUGFeRDTradeSettlementDebit' as 'IZUGFeRDTradeSettlement' and 'IZUGFeRDTradeSettlementPayment' return 'null' for 'getPaymentXML()' which is not allowed to be written to the XML file.