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Subject: ambiguity in current TAG draft
(a) is aligned with both our definiions of predicate and of prescription level in 2.1:
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
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