Re: [tag] ambiguity in current TAG draft

["Stephen Green" <stephen.green@documentengineeringservices.com>]

I agree the last example does not *need* a negative prescription level

>
>
> normative source: x MUST NOT do y
>
> target: x
>
> predicate: [target x] does y
>
> prescription level: not permitted
>

The above is correct use of the *negative* prescription level


>
>
> Which so far we do not endorse.
>
>
>
> while:
>
> normative source: x MUST NOT do y
>
> target: x
>
> predicate: [target x] does not do y
>
> prescription level: not permitted
>
>


This second example should instead be:

 normative source: x MUST NOT do y

 target: x

 predicate: [target x] does not do y

 prescription level: permitted



I would not want to push the argument in favour of 'allowing' or 'accommodating'
the negative prescription levels too much. However, the main argument in their
favor seems to me to be the fact that they do already exist as a standard in the
relevant RFC and this is probably for a good reason. After all, they
need not exist
in specifications either since the requirement could have been written


 normative source: x MUST not do y

But is there *ever* a subtle difference between

 normative source: x MUST not do y
 and
 normative source: x MUST NOT do y

?

I suspect there is but that to regard the MUST NOT as redundant in favor of
'MUST not' is probably not going to do much harm. It might be worth endorsing
a three-positive-value-only prescription level codelist just for the
benefit of being
able then to say that a predicate MUST always evaluate to 'true' when fuliflling
the normative requirement(s). So I'm happy to drop the argument since there is
some benefit in doing so and the benefit of keeping the 'MUST NOT' equivalent
Prescription Level and 'SHOULD NOT' equivalent is not easy to see or
demonstrate. However I have a feeling it does exist in more complex cases - but
that can be left to a more advanced treatment of TAs which might endorse the
corresponding extension of the prescription level list.

Best regards

Steve

-- 
Stephen D. Green

Document Engineering Services Ltd



http://www.biblegateway.com/passage/?search=matthew+22:37 .. and voice



2008/11/27 Durand, Jacques R. <JDurand@us.fujitsu.com>:
>
> I think we have an ambiguity in current TAG draft, revealed by the SCA use
> case:
>
> - When a normative statement is of the kind "x MUST NOT do y" , the natural
> way (in fact, the recommended way) to write the predicate is:
> "[target x] does not do y"
>
> Then, what should be the prescription level?
> (a) MANDATORY?
> (b) NOT PERMITTED?
>
>
> (a) is aligned with both our definiions of predicate and of prescription
> level in 2.1:
>
>    Predicate
>
> A predicate asserts, in the form of an expression, the feature (a behavior
> or a property) described in the referred specification statement(s). If the
> predicate is an expression which evaluates to "true" over the test assertion
> target, this means that the target exhibits this feature. "False" means the
> target does not exhibit this feature.
>
>    Prescription Level
>
> A keyword that qualifies how imperative it is that the requirement referred
> in Normative Source, be met. See possible keyword values in the Glossary.
>
>
>
> (b) is aligned with the Glossary definition of prescription level :
>
>
>
>     The test assertion defines a normative statement which may be mandatory
> (MUST/REQUIRED), not permitted (MUST NOT), permitted (MAY/OPTIONAL)
> preferred (SHOULD/RECOMMENDED) or not recommended (SHOULD NOT). This
> property can be termed its prescription level.
>
>
>
> The problem I see with allowing/recommending "negative" prescription levels,
> is that this entices the TA writer to write "negative" predicates (=true
> means violation), e.g.:
>
>
>
> normative source: x MUST NOT do y
>
> target: x
>
> predicate: [target x] does y
>
> prescription level: not permitted
>
>
>
> Which so far we do not endorse.
>
>
>
> while:
>
> normative source: x MUST NOT do y
>
> target: x
>
> predicate: [target x] does not do y
>
> prescription level: not permitted
>
>
>
> Would be confusing and sound like a double negation ("not permitted" for
> target to NOT do y)
>
>
>
> Do we really need negative prescription levels, and if yes how do they work?
>
>
>
> Jacques
>
>
>
>
>
>