preA :: (CTL m a s, Ord a, Ord s) => m -> Set s -> Set s
preA m y = Set.difference (states m) (preE m (Set.difference (states m) y))
preA :: (CTL m a s, Ord a, Ord s) => m -> Set s -> Set s
preA m y = Set.difference (states m) (preE m (Set.difference (states m) y))
preA m y = Set.difference (states m) (preE m (Set.difference (states m) y))