Start date of line period MUST be within invoice period.
ubl-invoice:Invoice[cac:InvoicePeriod/cbc:StartDate]/cac:InvoiceLine/cac:InvoicePeriod/cbc:StartDate | ubl-creditnote:CreditNote[cac:InvoicePeriod/cbc:StartDate]/cac:CreditNoteLine/cac:InvoicePeriod/cbc:StartDate
xs:date(text()) >= xs:date(../../../cac:InvoicePeriod/cbc:StartDate)