main :: [sys_message]
main = [Stdout (lay (map (show . take 30) [powers_of_3, evil, odious]))]

powers_of_3 :: [num]
powers_of_3 = map (popcount . (3^)) [0..]

evil :: [num]
evil = filter f [0..] where f n = popcount n mod 2 = 0

odious :: [num]
odious = filter f [0..] where f n = popcount n mod 2 = 1

popcount :: num -> num
popcount 0 = 0
popcount n = n mod 2 + popcount (n div 2)