help fuzzing/finding Horn CNF formula

[Georgi Guninski <guninski () guninski com>]

i know i am dumb.

i am looking for a HORN CNF that is SAT if and only if |x != y| ($x \ne y$) for boolean x,y.

using the Horn constraints in [1] (at most k '1's) + the sought inequality/negation, it might be possible to encode 
Exact Sat (XSAT) to fast Horn CNF (md5 preimage falls in this case)

i am not sure if such CNF formula exists though current academics claim Horn Sat is "P-complete", i.e. powerful enough 
for the formula.

note that the size of the CNF doesn't matter much (it cares about just 2 variables, the other vars are temp).

i failed fuzzing it :)

[1] Towards an Optimal CNF Encoding of Boolean Cardinality Constraints
C Sinz
http://www-sr.informatik.uni-tuebingen.de/~sinz/CardConstraints/cp2005.pdf

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