Allowance/charge ChargeIndicator value MUST equal 'true' or 'false'
ubl-invoice:Invoice/cac:AllowanceCharge | ubl-invoice:Invoice/cac:InvoiceLine/cac:AllowanceCharge | ubl-creditnote:CreditNote/cac:AllowanceCharge | ubl-creditnote:CreditNote/cac:CreditNoteLine/cac:AllowanceCharge
normalize-space(cbc:ChargeIndicator/text()) = 'true' or normalize-space(cbc:ChargeIndicator/text()) = 'false'