> Taking it out probably had something to do with NaNs, but this
> discussion needs to avoid getting lost in NaN paradoxes and instead
> focus on a notion of membership that is ALWAYS true given object
> identity.  This is an essential pragmatism necessary for reasoning 
about
> programs.

I agree wholeheartedly.  NaN comparison weirdness isn't anywhere near
important enough to justify breaking these invariants.  Though I imagine 
that if 'x == x' started returning True for NaNs there might be some 
complaints.

> P.S.  Mark, those Py2.6 invariants are not still true in Py3.0:

You're right, of course.