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Subject: Prescription levels

I may have said this before, but I'll say it again because it helps clarify what Prescription Levels can do in the larger context: the set of Prescription Levels must be considered extensible, so that a user of TAs can integrate TAs for conformance with TAs for other purposes, such as product requirements.

The relationship between the Predicate and the Prescription Level then becomes the following:
When the Predicate is true, the Prescription Level tells you what you can conclude about the product.

Examples: Predicate true on a TA that is Mandatory: product fulfills a spec requirement.
Predicate true on a TA that is Recommended: product exhibits a trait that is considered desirable. (If the predicate were false, you could not conclude that the product is "bad" on the basis of that TA alone.)
Product X has predicate true on a product-specific TA that is a version 1.5 required feature: Product X implements one of the features that version 1.5 needed to have. (Vendor-extension TA.)
Predicate true on a TA that is Must Not: product violates a spec requirement.

I see negative Prescription Levels as being useful for testable requirements that are widespread. For example, the requirements for a well-formed XML document, such as the requirement that no element name can contain a space, can pervade the testing of a product that produces XML documents. A TA to express "no element name can contain a space" may work better as a negative, so that the predicate can be "element name contains a space". Also consider the implications of a TA for some spec (not the XML spec itself) that requires the document to be well-formed XML. Somewhere, there needs to be a "grand union" TA that says a document is considered well-formed if it passes dozens of TAs that constitute the individual requirements for well-formed-ness. It might not be a simple AND relation of the dozens of TAs, but an AND of the ones with Mandatory prescription level plus a NOT of the OR of all the ones with a Must Not prescription level.
.................David Marston
IBM Research

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