action p x = if p x then succ x else x
fenceM p q s [] = guard (q s) >> return []
fenceM p q s (x:xs) = do
(f,g) <- p
ys <- fenceM p q (g s) xs
return $ f x ys
ncsubseq = fenceM [((:), action even), (flip const, action odd)] (>= 3) 0