For Danish suppliers PaymentMandate/ID and PayerFinancialAccount/ID are mandatory when payment means is 49
ubl-invoice:Invoice[$DKSupplierCountry = 'DK' and $DKCustomerCountry = 'DK']/cac:PaymentMeans
not((cbc:PaymentMeansCode = '49') and not((normalize-space(cac:PaymentMandate/cbc:ID/text()) != '') and (normalize-space(cac:PaymentMandate/cac:PayerFinancialAccount/cbc:ID/text()) != '')) )