A VAT exemption reason MUST be provided if the VAT category code is exempt or reverse charge.
//cac:TaxSubtotal[cac:TaxCategory/cac:TaxScheme/cbc:ID = 'VAT'](cac:TaxCategory/cbc:TaxExemptionReason) or not ((normalize-space(cac:TaxCategory/cbc:ID)='E') or (normalize-space(cac:TaxCategory/cbc:ID)='AE'))