data Stream a = (:+) a (Stream a)
instance (Num a) => Num (Stream a) where
    a0:+a' + b0:+b' = (a0+b0):+(a'+b')

rep :: a -> Stream a
rep a = a :+ rep a

drop :: Int -> Int -> Stream a -> Stream a
drop 0 k (a0:+(a1:+a'')) = a1 :+ drop (k-2) k a''
drop i k (a0:+a') = a0 :+ drop (i-1) k a'

sum :: Num a => Stream a -> Stream a
sum (a0:+a') = a0 :+ (sum a' + rep a0)

moessner :: Num a => Int -> Stream a
moessner n = foldr (.) id [sd k (k+1) | k <- [1..n]] ones