'Co-requisites' (co-constraints) cf 'pre-requisites'

[stephen.green@systml.co.uk]

In regard to following up the comment/question I made on the
call:

What I'm asking is - are there sometimes 'co-requisites' rather
than pre-requisites - that is, TAs which have to be taken
together to produce an outcome without any requirement about
which has to be tested first?

An example that springs to mind from UBL is the need to use
two passes of validation of coded values in an instance. Both
have to be passed for there to be any outcome. They are both
using different artefacts (an XML Schema and a Schematron schema
as it happens but that isn't relevant here). It isn't necessarily
the case that one test/validation has to be performed before
the other, as long as both are performed. (In this example there
is one of the tests which is meaningless if the other isn't
tested with a pass but that is not helpful to the example -
it could have been that that was not the case and that each is
irrelevant if the other isn't passed.) This is one of the main
UBL TAs if not THE main TA. There are other examples such as
the need to pass two sets of validation (no requirement which
goes first) when there is a second set of constraints applied
to a subset instance. Again both Schematron and XML Schema
would be typical artefacts to be applied to the IUT to
determine the outcome. Neither would be relevant without the other
in some situations. If the two TAs can't be combined in some way
(and making just one TA isn't really appropriate) then there would
be a need to add TA-like information to a conformance clause which
probably didn't belong there but is really TA material.

Other examples relate to things like tax requirements and are
more semantic than technical but they are still important in
specifying document vocabularies like UBL. If and only if an invoice
has any sales or value added tax it must also have a tax
registration number: for which there might be one TA to see
that it has tax correctly applied and another to see whether
or not it has a tax reg number:
if both then pass
if one only then fail
if neither then pass

Thanks

-- 
Stephen Green

Partner
SystML, http://www.systml.co.uk
Tel: +44 (0) 117 9541606

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