Someone wrote:
	> Seems to be a good idea to me. First I've thought of taking "self" as
	> the placeholder, but then the Set couldn't contain itself...
	
A Set cannot contain itself anyway.  One of the fundamental things about a
Set is that you shouldn't put anything in it whose hash value will change.
Even if you add a Set to itself as the very last change, the hash value
*before* the change will be used to place it, but afterwards its hash value
will be different.

    s := Set new.
    s hash
=>  2589
    s add: s
    s hash
=> WHOOPS, INFINITE LOOP.

In fact, a *lot* of Squeak collections assume they are trees, not graphs.
    a := Array new: 1.
    a at: 1 put: a.
    a hashy
=> WHOOPS, INFINITE LOOP.