open monad io unsafe.cell unsafe.console

liftM2 f m1 m2 = do
  x1 <- m1
  x2 <- m2
  return (f x1 x2)

a k x1 x2 x3 x4 x5 = do
  r <- return $ ref k
  let b = & do k <- return $ pred (valueof r)
             a k b x1 x2 x3 x4
  if k <= 0 then liftM2 (+) x4 x5 else b

_ = a 10 (!!1) (!! -1) (!! -1) (!!1) (!!0) >>= (putStr << show) ::: IO
  where (!!) f = & return f ::: IO