sudan n x y | n == 0    = trace (x + y)
            | y == 0    = trace x
            | otherwise = do
  trace 0
  sudan' <- sudan n x (y - 1)
  sudan (n - 1) sudan' (sudan' + y)
    where
      trace r = do
        putStrLn("sudan " ++ show n ++ " " ++ show x ++ " " ++ show y)
        return r

sudTup (n, x, y) = do { p <- sudan n x y; return p }
sudList [n, x, y] = do { q <- sudTup (n, x, y); return q }

main = do
  r <- sudList [1, 5, 4]
  print r