Hashing is just an optimization trick, albeit a good one.
I suppose the real issue about sets is, independent of hashing issues,

should:

	Set new = Set new

How about:

	Array new = Array new

(A) What is the general principal for deciding whether objects should be
"identity objects"?

(B) What is the proper way to indicate such a decision?

The easy answer to (B) is the existence of an #= method in the class.
Question (A) is much more interesting.

Identity objects:
	Objects that are equal (=) only to objects are identical (==)
	
-- Mike Klein