My 2 cents on x removeAll: y.

I'd expect the argument y to be left as it is, while the receiver x now
doesn't contain any element that is in y.

When x == y, we have:

I'd expect x to be left as it is, while x now doesn't contain any element
that is in x.

In other words, I'd expect x to be left alone *and* to be empty at the same
time.  This only works on empty collections.

We arrive at this nonsense because the collections are identical.  Similar
arguments apply to addAll: and maybe others.

The expressions x removeAll: x and x addAll: x are not a special case or an
error condition: they are NONSENSE.  If they occur, the real problem is that
the intentions of the program are nonsense as written by the developer.

Andres.
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