preE :: (CTL m a s, Ord a, Ord s) => m -> Set s -> Set s
preE m y = Set.filter (\ s -> exists (`Set.member` next m s) y) (states m)
preE :: (CTL m a s, Ord a, Ord s) => m -> Set s -> Set s
preE m y = Set.filter (\ s -> exists (`Set.member` next m s) y) (states m)
preE m y = Set.filter (\ s -> exists (`Set.member` next m s) y) (states m)