[PATCH] Xor reassociation

[Sean Silva]

FYI, it's really easy to use a theorem prover to show these kinds of
identities.

Microsoft Research's z3 theorem prover has a convenient web interface that
allows creating permalinks that are easy to share as "proof". It is also
well documented (see <http://rise4fun.com/Z3Py/tutorial/guide>; the "Bit
Tricks" section is especially relevant).

For example, here is a proof of the 4 identities you showed: <
http://rise4fun.com/Z3Py/slF3A>.

Also, z3 will give you a counterexample if the identity is not correct.

-- Sean Silva
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