Pattern matcher: NotLink ChoiceLink not working

[linas]

The expression (NotLink (ChoiceLink single-atom)) should be equivalent to (AbsentLink ingle-atom) but will almost surely fail in the current implementation. Note this is closely related to issue #215

Just like issue #215, this will probably be hard (?) to implement, because absent links really behave differently than other things: they are a kind of virtual clause, which can only be evaluated after a grounding is found.

This is marked "enhancement" becausse its low-priority; it is not blocking any work, and has reasonable work-arounds .. i.e. just use AbsentLink!

[linas]

Currently, (NotLink (ChoiceLink xxx)) throws an exception n the classserver, because ChoiceLink is not a boolean type, and thus, taking it's negation is undefined.

I guess the intended semantics here is that ChoiceLink should evaluate to whichever choice was taken, else empty-set if none were taken. Then Not would mean is-empty/is-not-empty. Perhaps we should have a boolean EmptyLink that evaluates to true, when it is empty, else evaluates to false? Thus, EmptyLink behaves a lot like PresentLink, except that it does not quote the Choice.