Missing rule in UBL for checking for the mandatory CompanyID (VAT-id) in PartyTaxScheme

[MartinForsberg-Ecru]

There is a missing rule for the UBL syntax binding that makes it possible to provide a PartyTaxScheme without a CompanyID.

This goes unnoticed:

	<cac:PartyTaxScheme>
<!--		<cbc:CompanyID>SE123456789001</cbc:CompanyID> -->
		<cac:TaxScheme>
			<cbc:ID>VAT</cbc:ID>
		</cac:TaxScheme>
	</cac:PartyTaxScheme>	

A rule along the following lines would solve the issue:

<rule context="cac:PartyTaxScheme">
	<assert test="exists(cac:TaxScheme/cbc:ID) and exists(cbc:CompanyID)" flag="fatal" id="UBL-SR-nnn">[UBL-SR-nnn]- CompanyID (VAT Identifier) must be stated when providing the PartyTaxScheme/TaxScheme/ID.</assert>
</rule>