Opaque Expressions for DataUses

When data structures and conditions are underspecified, it is useful to be able to express conditions informally, which can then be refined as the tests become more fully specified. Syntactically, a keyword (or set of keywords) shall be used to express the use of the opaque expressions, and a string should be used to describe the expression. Specialised annotations would be a suitable approach.

For example: Collection size check too specific for TP, perhaps we could have informal assertions to be used inside TPDs, e.g.:

receive STRUCT(f1 = instance_1, f2 = <keyword> "Collection not empty" ?, f3 = …)

Where is any of 'ensure' | 'check' | 'indicating value' | 'validate' or similar

This is also a language feature (link with Mantis)

added feature label

Initial proposal: check/where annotation, restricted to AnyValue and LiteralValueUse for the time being, depending also on syntactical feasibility (rationale: if you already have a specific data structure then you don't need opaque expression anymore), e.g.:

receive STRUCT(f1 = instance_1, f2 = ? {where: "Collection not empty"}, f3 = …)

In case the braces and colon may be skipped and well (currently aligned with constraints).

Need to update parts 1 and 8, possibly also 2.

changed the description

Added to DataElementUse as well (provisional)