Obfuscated SWAP: not portable!

[Richard H. Gumpertz]

"x ^= y ^= x ^= y" differs from "x = y = z = 0" in that the former makes TWO
assignments to x.  Side effects (i.e., assignments to variables) need only be
completed at "sequence points"; between sequence points, any order of
evaluation (including assignment operators) is allowed!

The following are all legal interpretations of the sequence in question,
yet may yield different results:
      (t = x ^= y, x ^= y ^= t)
      (t = x, y ^= x ^= y, x = t ^ y)
      (t = x ^ y, x ^= y ^= t, x = t)
I am sure there are other interpretations as well.