The following theorem is obviously true, but how is it proved (most cleanly and simply) in Haskell?

Theorem:  (nondecreasing xs) => nondecreasing (insert x xs), where:

   nondecreasing               :: (Ord a) => [a] -> Bool
   nondecreasing []            =  True
   nondecreasing xxs@(x : xs)  =  and [a <= b | (a, b) <- zip xxs xs]

   insert                      :: (Ord a) => a -> [a] -> [a]
   insert x xs                 =  takeWhile (<= x) xs ++ [x] ++ dropWhile (<= x) xs

Thanks.

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