[m-dev.] Constraint goals in negated contexts: a solution

[Ralph Becket]

I believe I have a palatable solution to the problem of constraint goals
appearing in negations and the conditions of if-then-else goals.

* Ask goals must be declared impure because they allow one to break
referential transparency (they can give different answers depending upon
the execution history of the program).

* Tell goals are pure, but should be illegal in a (pure) negated context
because the compiler doesn't know how to assert the negation of the tell
outside the negated context.

* Therefore, any goal with a non-local inst-any variable in a negated
context should be implicitly impure.  (Conceivably a tell goal could
appear in a negated context and still be pure, but it would be
unreasonable to expect the compiler to decide this for itself.)

I think this fixes the problem philosophically and practically.

Any problems?
-- Ralph
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