tail recursive list concat

(* revappend takes two 'a lists as arguments and concatenates them
 * with the side effect of reversing the order of the first list. *)
let rec revappend l1 l2 =
   match l1 with
   | [] -> l2
   | h :: t -> revappend t (h::l2)

(* reverse a list *)
let reverse l = revappend l []

(* Concatonate 'a list list -> 'a list should be tail recursive*)
let concat ll =
   let rec aux ll l =
      match ll with
      | [] -> l
      | h :: t -> aux t (revappend h l)
   in reverse (aux ll [])