missing 'xor' Boolean operator

[greg]

Terry Reedy wrote:

> In Math and Python, a<b<c means a<b and b<c, not (a<b)<c or a<(b<c).
> != is a comparison operator like <,

Although Python extends the chaining principle to
!=, this is somewhat questionable, because
a < b and b < c implies a < c, but a != b and
b != c does not imply a != c.

I'm not sure I've ever seen a mathematician
write a != b != c, but if I did, I would tend
to think he meant to say that none of a, b,
c are equal to any other. That's not what it
means in Python, though.

-- 
Greg