如何在quickCheck中使用'oneof'(Haskell)

import Test.QuickCheck

data Tree = Empty | Node Int Tree Tree deriving (Show)

genTree :: Gen Tree
genTree = sized $ \n ->
  if n == 0
    then return Empty
    else do
      leftSize <- choose (0, n - 1)
      let rightSize = n - leftSize - 1
      root <- arbitrary
      left <- genTree leftSize
      right <- genTree rightSize
      return (Node root left right)

main = quickCheck $ \n ->
  let tree = runGen genTree n
  in checkTree tree

checkTree :: Tree -> Bool
checkTree Empty = True
checkTree (Node x left right) =
  x >= maxLeft && x >= maxRight &&
  checkTree left && checkTree right
  where
    maxLeft = case left of
      Empty -> -1
      Node x _ _ -> x
    maxRight = case right of
      Empty -> -1
      Node x _ _ -> x

oneof :: [Gen a] -> Gen a