On Fri Feb 06 2015 at 12:12:19 AM Steven D'Aprano <steve@pearwood.info> wrote:
I think there are plenty of other major design decisions to be made.
What's the union between a regular set and an ordered set? Is there a
frozen ordered set? Do ordered sets compare unequal if they differ only
in order (like lists)?

Are there many options for the union question? I think the only sane choice for any union involving ordered sets is an unordered set - in any other case the resulting order would be wrong or at least not obviously right, and that's almost certainly better expressed explicitly.

Ed Kellett