Start date of line period MUST be within invoice period.
PEPPOL-EN16931-R110pxs-taxdata:TaxData/pxs-taxdata:ReportedTransaction/pxs-taxdata:ReportedDocument[cac:InvoicePeriod/cbc:StartDate]/pxs-taxdata:DocumentLine/cac:InvoicePeriod/cbc:StartDatexs:date(text()) >= xs:date(../../../cac:InvoicePeriod/cbc:StartDate)