This is proceeding as a series of discussions with Nate Lust; I'll consider it complete when he's fully ready to write up the SpanSet design proposal for broader review. I think there's currently only minor issue to be resolved (set operations that return new nontrivial {{InverseSpanSet}}s).

Actual work has been performed on Nate Lust's issues. This issue just captures Jim Bosch's effort on that, which has been spread out over several of those. I believe that design work is now completed, and it's just implementation from here on.