list sum

let rec revZipWithDefault accum dval lsta lstb =
    match lsta with
    | ha :: ta ->
            (match lstb with
            | hb :: tb -> zipWithDefault ((ha, hb) :: accum) dval ta tb
            | _ -> zipWithDefault ((ha, dval) :: accum) dval ta [])
    | _ -> 
            (match lstb with
            | hd :: tl -> zipWithDefault ((dval, hd) :: accum) dval [] tl
            | _ -> accum);;

let zipInts a b = revZipWithDefault [] 0 a b |> List.rev;;
zipInts [1;2;3] [4;5] |> map (fun (a, b) -> a + b);;