diff --git a/sequence.ml b/sequence.ml
index 396e6fe..d322fe3 100644
--- a/sequence.ml
+++ b/sequence.ml
@@ -133,101 +133,89 @@ let intersperse elem seq =
     let first = ref true in
     seq (fun x -> (if !first then first := false else k elem); k x)
 
-(** Intermediate storage for elements of a sequence. The most important
-    features are:
-    - easy iteration
-    - fast insertion at the end
-    - memory efficiency
-    The actual implementation is currently a kind of 2-3 B-tree. *)
+(** Mutable unrolled list to serve as intermediate storage *)
 module MList = struct
-  type 'a t =
-  | Node2 of 'a t * 'a t          (** 2 children *)
-  | Node3 of 'a t * 'a t * 'a t   (** 3 children *)
-  | Leaf of int ref * 'a array    (** Leaf (with content) *)
+  type 'a t = {
+    content : 'a array;   (* elements of the node *)
+    mutable len : int;    (* number of elements in content *)
+    mutable tl : 'a t;    (* tail *)
+  } (** A list that contains some elements, and may point to another list *)
+
+  let _empty () : 'a t = Obj.magic 0
+    (** Empty list, for the tl field *)
 
-  (** Empty list *)
   let make n =
-    assert (n > 1);
-    Leaf ((ref 0), Array.make n (Obj.magic 0))
+    assert (n > 0);
+    { content = Array.make n (Obj.magic 0);
+      len = 0;
+      tl = _empty ();
+    }
 
   let rec is_empty l =
-    match l with
-    | Node2 (a, b) -> is_empty a && is_empty b
-    | Node3 (a, b, c) -> is_empty a && is_empty b && is_empty c
-    | Leaf (n, content) -> !n = 0
+    l.len = 0 && (l.tl == _empty () || is_empty l.tl)
 
-  (** Iterate on the elements, in insertion order *)
   let rec iter f l =
-    match l with
-    | Node2 (a, b) -> iter f a; iter f b
-    | Node3 (a, b, c) -> iter f a; iter f b; iter f c
-    | Leaf (n, content) -> for i = 0 to !n - 1 do f content.(i); done
+    for i = 0 to l.len - 1 do f l.content.(i); done;
+    if l.tl != _empty () then iter f l.tl
 
   let iteri f l =
-    let r = ref 0 in
-    iter (fun x -> f !r x; incr r) l
+    let rec iteri i f l =
+      for j = 0 to l.len - 1 do f (i+j) l.content.(j); done;
+      if l.tl != _empty () then iteri (i+l.len) f l.tl
+    in iteri 0 f l
 
-  (** Iterate on the elements, in reverse insertion order *)
   let rec iter_rev f l =
-    match l with
-    | Node2 (a, b) -> iter_rev f b; iter_rev f a
-    | Node3 (a, b, c) -> iter_rev f c; iter_rev f b; iter_rev f a
-    | Leaf (n, content) -> for i = !n - 1 downto 0 do f content.(i); done
+    (if l.tl != _empty () then iter_rev f l.tl);
+    for i = l.len - 1 downto 0 do f l.content.(i); done
 
-  (** Number of stored elements *)
   let length l =
-    let rec len l = match l with
-    | Node2 (a, b) -> len a + len b
-    | Node3 (a, b, c) -> len a + len b + len c
-    | Leaf (n, _) -> !n
-    in len l
+    let rec len acc l =
+      if l.tl == _empty () then acc+l.len else len (acc+l.len) l.tl
+    in len 0 l
 
-  (** Get element by index, or raise Not_found *)
-  let get l i =
-    let r = ref None in
-    (* return number of elements traversed, or raise Exit *)
-    let rec get l i =
-      match l with
-      | Leaf (n, content) when i < !n ->
-        r := Some (content.(i));
-        raise Exit
-      | Leaf (n, _) -> i - !n
-      | Node2 (a, b) -> let i' = get a i in get b i'
-      | Node3 (a, b, c) -> let i' = get a i in let i'' = get b i' in get c i''
-    in
-    (* traverse, and check whether the element has been found *)
-    ignore (get l i);
-    match !r with
-    | None -> raise Not_found
-    | Some x -> x
+  (** Get element by index *)
+  let rec get l i =
+    if i < l.len then l.content.(i)
+    else if i >= l.len && l.tl == _empty () then raise (Invalid_argument "MList.get")
+    else get l.tl (i - l.len)
 
-  (** Push [x] at the end of the storage. Returns the new tree (may be the
-      same), in other cases this modifies [l] in place. *)
+  (** Push [x] at the end of the list. It returns the block in which the
+      element is inserted. *)
   let rec push x l =
-    match l with
-    | Node2 (a, b) ->
-      begin
-        match push x b with
-        | Node2 (b1, b2) -> Node3 (a, b1, b2)  (* merge *)
-        | b' -> Node2 (a, b')
+    if l.len = Array.length l.content
+      then begin (* insert in the next block *)
+        (if l.tl == _empty () then l.tl <- make (Array.length l.content));
+        push x l.tl
+      end else begin  (* insert in l *)
+        l.content.(l.len) <- x;
+        l.len <- l.len + 1;
+        l
       end
-    | Node3 (a, b, c) ->
-      let c' = push x c in
-      Node3 (a, b, c')  (* insert in rightmost tree *)
-    | Leaf (n, content) ->
-      if !n < Array.length content
-        then (content.(!n) <- x; incr n; l) (* insert in array *)
-        else
-          let len = !n + 2 in  (* increase a bit length *)
-          let l' = Leaf (ref 0, Array.make len (Obj.magic 0)) in
-          Node2 (l, push x l')  (* insert in new leaf *)
+
+  (** Reverse list (in place), and returns the new head *)
+  let rev l =
+    let rec rev prev l =
+      (* reverse array *)
+      for i = 0 to (l.len-1) / 2 do
+        let x = l.content.(i) in
+        l.content.(i) <- l.content.(l.len - i - 1);
+        l.content.(l.len - i - 1) <- x;
+      done;
+      (* reverse next block *)
+      let l' = l.tl in
+      l.tl <- prev;
+      if l' == _empty () then l else rev l l'
+    in
+    rev (_empty ()) l
 
   (** Build a MList of elements of the Seq. The optional argument indicates
       the size of the blocks *)
   let of_seq ?(size=8) seq =
-    let l = ref (make size) in
+    (* read sequence into a MList.t *)
+    let start = make size in
+    let l = ref start in
     seq (fun x -> l := push x !l);
-    !l
+    start
 end
 
 (** Iterate on the sequence, storing elements in a data structure.
@@ -461,9 +449,14 @@ let of_stream s =
 
 (** Convert to a stream. The sequence is made persistent. *)
 let to_stream seq =
-  let l = MList.of_seq seq in
-  Stream.from
-    (fun i -> try Some (MList.get l i) with Not_found -> None)
+  let l = ref (MList.of_seq seq) in
+  let i = ref 0 in
+  let rec get_next () =
+    if !l == MList._empty () then None
+    else if (!l).MList.len = !i then (l := (!l).MList.tl; i := 0; get_next ())
+    else let x = (!l).MList.content.(!i) in (incr i; Some x)
+  in
+  Stream.from (fun _ -> get_next ())
 
 (** Push elements of the sequence on the stack *)
 let to_stack s seq = iter (fun x -> Stack.push x s) seq