Introduce SetConnectorLink (or such) as parent type of UnionLink, etc

[ngeiswei]

When matching classes of atoms, it could be convenient to be able to agglomerate all atom types representing concepts/sets. So for instance

(Get (TypeInh X (Type 'SetConnector)) X)

would match Set, Union, Intersection, Complement and possibly more.

[linas]

Sure. Except that SetConnector feels non-standard. Perhaps SetOperator or SetOperation would be better. Maybe SetFunction (Saying SetValuedFunction seems like too long a name).

However, saying that Set inherits from SetOperation seems awkward, so perhaps Collection is a better name. That is, Set, Union, etc. are all Collection s.

So I nominate Collection - that would be my favorite.

[linas]

This is a follow-up to #2844