OCaml:和关键字语法错误

我已经编写了这个mergesort实现,如果我将divide函数放在mergesort函数之外,它会很好地工作。但是,当我试图将divide作为mergesort的内部函数时,我遇到了一个语法错误。

我知道,一定有一些非常简单的解释。我在网上找遍了,但什么也没找到。

let mergesort list = 
  let rec sort lists acc = (
    let rec merge sublist1 sublist2 merged_list =
    match sublist1 with
      |[] -> merged_list @ sublist2
      |hd1 :: tl1 -> 
        match sublist2 with
          |[] -> merged_list @ sublist1
          |hd2 :: tl2 -> 
            if hd1 < hd2 then merge tl1 sublist2 (merged_list @ hd1::[])
            else merge sublist1 tl2 (merged_list @ hd2::[])
    in match lists with 
      |[] -> 
        (match acc with
          |[] -> []
          |hd :: [] -> hd
          |_ -> sort acc [])
      |hd :: tl -> sort (List.tl tl) ((merge (List.hd tl) hd [])::acc)  
  )
  and rec divide list list_of_lists = (
    match list with
      [] -> list_of_lists
      |hd :: tl -> divide tl ((hd :: []) :: list_of_lists)
  )
  in sort (divide list []) []
;; 

结果是:

Characters 567-570:
and rec divide list list_of_lists = (
    ^^^
Error: Syntax error