subsequencesOfSize :: Int -> [a] -> [[a]]
subsequencesOfSize n xs = let l = length xs
                          in if n>l then [] else subsequencesBySize xs !! (l-n)
 where
   subsequencesBySize [] = [[[]]]
   subsequencesBySize (x:xs) = let next = subsequencesBySize xs
                               in zipWith (++) ([]:next) (map (map (x:)) next ++ [[]])
 
main = do
	print $ length $ subsequencesOfSize 3   [1..350]
	print $ length $ subsequencesOfSize 147 [1..150]

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