(* A version that doesn't hide swb *)
(* swb = subtract with borrow *)
fun swb b []      []  = if b=0 then [] else [~b]
  | swb b l       []  = swb b l [0]
  | swb b []      r   = swb b [0] r
  | swb b (l::ls) (r::rs) =
    let
        val digit = l - b - r;
    in
        if digit >= 0 then
            digit :: (swb 0 ls rs)
        else
            (digit + 10) :: (swb 1 ls rs)
    end;

fun subDigitList l r = swb 0 l r;

(* a better version that does hide swb using local *)
local
    fun swb b []      []  = if b=0 then [] else [~b]
      | swb b l       []  = swb b l [0]
      | swb b []      r   = swb b [0] r
      | swb b (l::ls) (r::rs) =
        let
            val digit = l - b - r;
        in
            if digit >= 0 then
                digit :: (swb 0 ls rs)
            else
                (digit + 10) :: (swb 1 ls rs)
        end;
in
     fun subDigitList l r = swb 0 l r;
     (* could also have written it as:
     val subDigitList = swb 0;*)
end;