[Okasaki, C. DR EECS]

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