cat2 :: [Integer]
cat2 = scanl (\c n -> c * 2 * (2 * n - 1) `div` (n + 1)) 1 [1 ..]
cat2 :: [Integer]
cat2 = scanl (\c n -> c * 2 * (2 * n - 1) `div` (n + 1)) 1 [1 ..]
cat2 = scanl (\c n -> c * 2 * (2 * n - 1) `div` (n + 1)) 1 [1 ..]