[My sincere apologies for the multiple posts. I forgot to include the subject line on my previous post, which makes it unclickable in the archive...] I wrote:

You've just highlighted why the collections hierarchy in Edison was a lattice of 8 classes. Basically, there are two choices in each of three different dimensions:

1. The set/map distinction 2. Require Ord or don't (your first point above) 3. "Observable" or not (your second point above)

Oops, I typed too fast. Edison actually has two choices in each of four different dimensions. The first is the set/map distinction, but those are in two separate class hierarchies. Within each of those class hierarchies, the three dimensions are Ord/not, Observable/not, and "unique"/not, where the last dimension is the difference between sets and bags. -- Chris