Since I'm learning OCaml, I've been writing little functions as I get a 
grasp on each little bit of the syntax.  One of these functions is 
"permute", which takes a string and returns a list of all the permutations 
of the string.  Here's the function:

let rec permute_rec f s =
  if String.length s = 1 then
    (f ^ s) :: []
  else
    let l = ref [] in
    for i = 0 to ((String.length s) - 1) do
      let c = String.sub s i 1 in
      let rem = (String.sub s 0 i) ^ ((String.sub s (i+1) ((String.length 
s)-i-1))) in
	l := (List.append !l (permute_rec (f^c) rem))
    done;
      !l;;

let permute = permute_rec "";;

Now, I'm sure this isn't the best way to code that function.  What I'm 
hoping is that someone can show me a few things about the function: how to 
make it tail recursive; a better (less imperative?) method of writing it, 
and which parts are not consistent with common/standard OCaml idiom.  
*Anything* I'm shown should at least help me learn, so if you have any 
comments/opinions, please tell me!

Thanks,
Jeremy
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