If both supplier and customer are located in Germany, and if "Invoice type code" (BT-3) contains the code 384 (Corrected invoice), "PRECEDING INVOICE REFERENCE" (BG-3) should be provided at least once.
(/ubl-invoice:Invoice | /ubl-creditnote:CreditNote)[$supplierCountryIsDE and $customerCountryIsDE]((not(normalize-space(cbc:InvoiceTypeCode) = '384' or normalize-space(cbc:CreditNoteTypeCode) = '384') or (cac:BillingReference/cac:InvoiceDocumentReference)))