[ibr-co-20]-If Invoice line period (ibg-26) is used, the Invoice line period start date (ibt-134) or the Invoice line period end date (ibt-135) MUST be filled, or both.
ibr-co-20
cac:InvoiceLine/cac:InvoicePeriod | cac:CreditNoteLine/cac:InvoicePeriod
exists(cbc:StartDate) or exists(cbc:EndDate)