regression/down.sml

   1 val rec down =
   2    fn 0 => 0
   3     | n => 1 + down' (n - 1) + down (n - 1)
   4 and down' =
   5    fn 0 => 0
   6     | n => 1 + down (n - 1) + down' (n - 1)
   7
   8 val _ = down 13