Should we allow empty collections?

[uscholdm]

If members are added or removed from a collection and there are no members at a given time but some will be added, you have an empty collection. We don't want to have it cease to be a collection.

[rjyounes]

Is this a case we are likely to encounter in our modeling domains, or is it strictly theoretical?

If we allow empty collections, then the restriction on Collection has to be removed. In that case the class has no formal semantics.

[rjyounes]

Duplicates #570.