For the Greek Suppliers, the VAT must start with 'EL' and must be a valid TIN number
cac:AccountingSupplierParty[$isGreekSender]/cac:Party/cac:PartyTaxScheme[normalize-space(cac:TaxScheme/cbc:ID) = 'VAT']/cbc:CompanyID
substring(.,1,2) = 'EL' and u:TinVerification(substring(.,3))