The VAT category tax amount MUST be zero if the VAT category code is reverse charge (since there is only one VAT category allowed it follows that the invoice tax total for reverse charge invoices is zero)
//cac:TaxTotal[cac:TaxSubtotal/cac:TaxCategory/cbc:ID = 'AE']every $taxamount in cac:TaxSubtotal/cbc:TaxAmount satisfies $taxamount = 0