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Predicate vote_up/2
 vote(?PostId, ?Vote) is nondet[private]
 vote_up(?PostId, ?Vote) is nondet[private]
 vote_down(?PostId, ?Vote) is nondet[private]
True when PostId has been voted with Vote. Vote is either 1 or -1. The predicates vote_up/2 and vote_down/2 only count up or down votes.