When Supplier is Greek, there must be one MARK Number
/ubl-invoice:Invoice[$isGreekSender and ( /*/cac:AccountingSupplierParty/cac:Party/cac:PostalAddress/cac:Country/cbc:IdentificationCode = 'GR')] | /ubl-creditnote:CreditNote[$isGreekSender and ( /*/cac:AccountingSupplierParty/cac:Party/cac:PostalAddress/cac:Country/cbc:IdentificationCode = 'GR')]
count(cac:AdditionalDocumentReference[cbc:DocumentDescription = '##M.AR.K##'])=1