f# - F# での "==>" 構文の意味

Question

F# で次のコードを検討してください。

let rec ordered xs = 
    match xs with
      | [] | [_]        -> true
      | x1 :: x2 :: xs'  -> x1 <= x2 && ordered (x2 :: xs')

その後

let rec insert x xs = 
    match xs with
    | []      -> [x]
    | y :: ys -> if x <= y then x :: y :: ys 
                 else           y :: insert x ys

そして最後に

let insertKeepsOrder (x : int) xs = ordered xs ==> ordered (insert x xs)

私が理解できないのは==>、最後の行の意味です!!!

それは何ですか？