diff --git a/src/data/CCDeque.ml b/src/data/CCDeque.ml
index 065fcb64..255c9fbf 100644
--- a/src/data/CCDeque.ml
+++ b/src/data/CCDeque.ml
@@ -3,23 +3,23 @@
 (** {1 Imperative deque} *)
 
 type 'a cell =
-  | Zero
   | One of 'a
   | Two of 'a * 'a
   | Three of 'a * 'a * 'a
   (** A cell holding a small number of elements *)
 
-type 'a node = {
+type 'a inner_node = {
   mutable cell : 'a cell;
-  mutable next : 'a node;
-  mutable prev : 'a node;
+  mutable next : 'a inner_node;
+  mutable prev : 'a inner_node;
 }
+
+type 'a node = Empty | Node of 'a inner_node
 (** Linked list of cells.
 
     invariant: only the first and last cells are allowed to
     be anything but [Three] (all the intermediate ones are [Three])
-    The first and last cell are [Zero] if and only if the
-    deque is empty *)
+    *)
 
 type 'a t = {
   mutable cur : 'a node;
@@ -48,12 +48,10 @@ type 'a t = {
 exception Empty
 
 let create () =
-  let rec cur = { cell=Zero; prev=cur; next=cur } in
-  { cur; size=0 }
+  { cur = Empty; size=0 }
 
 let clear q =
-  let rec cur = { cell=Zero; prev=cur; next=cur } in
-  q.cur <- cur;
+  q.cur <- Empty;
   q.size <- 0;
   ()
 
@@ -69,48 +67,54 @@ let clear q =
 let incr_size_ d = d.size <- d.size + 1
 let decr_size_ d = d.size <- d.size - 1
 
-let is_zero_ n = match n.cell with
-  | Zero -> true
-  | One _
-  | Two _
-  | Three _ -> false
-
 let bool_eq (a : bool) b = Stdlib.(=) a b
 
 let is_empty d =
   let res = d.size = 0 in
-  assert (bool_eq res (is_zero_ d.cur));
+  assert (bool_eq res (d.cur = Empty));
   res
 
+(*let rec cur = { cell=Zero; prev=cur; next=cur } in*)
 let push_front d x =
   incr_size_ d;
-  match d.cur.cell with
-    | Zero -> d.cur.cell <- One x
-    | One y -> d.cur.cell <- Two (x, y)
-    | Two (y, z) -> d.cur.cell <- Three (x,y,z)
-    | Three _ ->
-      let node = { cell = One x; prev = d.cur.prev; next=d.cur; } in
-      d.cur.prev.next <- node;
-      d.cur.prev <- node;
-      d.cur <- node (* always point to first node *)
+  match d.cur with
+    | Empty ->
+        let rec node = { cell=One x; prev = node; next = node } in
+        d.cur <- Node node
+    | Node n ->
+      match n.cell with
+        | One y -> n.cell <- Two (x, y)
+        | Two (y, z) -> n.cell <- Three (x,y,z)
+        | Three _ ->
+          let node = { cell = One x; prev = n.prev; next = n; } in
+          n.prev.next <- node;
+          n.prev <- node;
+          d.cur <- Node node (* always point to first node *)
 
 let push_back d x =
   incr_size_ d;
-  let n = d.cur.prev in (* last node *)
-  match n.cell with
-    | Zero -> n.cell <- One x
-    | One y -> n.cell <- Two (y, x)
-    | Two (y,z) -> n.cell <- Three (y, z, x)
-    | Three _ ->
-      let elt = { cell = One x; next=d.cur; prev=n; } in
-      n.next <- elt;
-      d.cur.prev <- elt
+  match d.cur with
+    | Empty ->
+        let rec node = { cell=One x; prev = node; next = node } in
+        d.cur <- Node node
+    | Node cur ->
+      let n = cur.prev in (* last node *)
+      match n.cell with
+        | One y -> n.cell <- Two (y, x)
+        | Two (y,z) -> n.cell <- Three (y, z, x)
+        | Three _ ->
+          let elt = { cell = One x; next=cur; prev=n; } in
+          n.next <- elt;
+          cur.prev <- elt
 
-let peek_front_opt d = match d.cur.cell with
-  | Zero -> None
-  | One x -> Some x
-  | Two (x,_) -> Some x
-  | Three (x,_,_) -> Some x
+let peek_front_opt d =
+  match d.cur with
+    | Empty -> None
+    | Node cur ->
+      match cur.cell with
+        | One x -> Some x
+        | Two (x,_) -> Some x
+        | Three (x,_,_) -> Some x
 
 let peek_front d = match peek_front_opt d with
   | None -> raise Empty
@@ -135,12 +139,13 @@ let peek_front d = match peek_front_opt d with
 *)
 
 let peek_back_opt d =
-  if is_empty d then None
-  else match d.cur.prev.cell with
-    | Zero -> assert false
-    | One x -> Some x
-    | Two (_,x) -> Some x
-    | Three (_,_,x) -> Some x
+  match d.cur with
+    | Empty -> None
+    | Node cur ->
+      match cur.prev.cell with
+        | One x -> Some x
+        | Two (_,x) -> Some x
+        | Three (_,_,x) -> Some x
 
 let peek_back d = match peek_back_opt d with
   | None -> raise Empty
@@ -164,10 +169,9 @@ let peek_back d = match peek_back_opt d with
 *)
 
 let take_back_node_ n = match n.cell with
-  | Zero -> assert false
-  | One x -> n.cell <- Zero; x
-  | Two (x,y) -> n.cell <- One x; y
-  | Three (x,y,z) -> n.cell <- Two (x,y); z
+  | One x -> (true, x)
+  | Two (x,y) -> n.cell <- One x; (false, y)
+  | Three (x,y,z) -> n.cell <- Two (x,y); (false, z)
 
 let remove_node_ n =
   let next = n.next in
@@ -175,20 +179,24 @@ let remove_node_ n =
   next.prev <- n.prev
 
 let take_back_opt d =
-  if is_empty d then None
-  else if Stdlib.(==) d.cur d.cur.prev
-  then (
-    (* only one cell *)
-    decr_size_ d;
-    Some (take_back_node_ d.cur)
-  ) else (
-    let n = d.cur.prev in
-    let x = take_back_node_ n in
-    decr_size_ d;
-    (* remove previous node *)
-    if is_zero_ n then remove_node_ n;
-    Some x
-  )
+  match d.cur with
+    | Empty -> None
+    | Node cur ->
+      if Stdlib.(==) cur cur.prev
+      then (
+        (* only one cell *)
+        decr_size_ d;
+        let is_zero, x = take_back_node_ cur in
+        if is_zero then d.cur <- Empty;
+        Some x
+      ) else (
+        let n = cur.prev in
+        let is_zero, x = take_back_node_ n in
+        decr_size_ d;
+        (* remove previous node *)
+        if is_zero then remove_node_ n;
+        Some x
+      )
 
 let take_back d = match take_back_opt d with
   | None -> raise Empty
@@ -202,32 +210,35 @@ let take_back d = match take_back_opt d with
 *)
 
 let take_front_node_ n = match n.cell with
-  | Zero -> assert false
-  | One x -> n.cell <- Zero; x
-  | Two (x,y) -> n.cell <- One y; x
-  | Three (x,y,z) -> n.cell <- Two (y,z); x
+  | One x -> (true, x)
+  | Two (x,y) -> n.cell <- One y; (false, x)
+  | Three (x,y,z) -> n.cell <- Two (y,z); (false, x)
 
 (*$T
   let q = of_list [1;2;3] in take_front q = 1 && to_list q = [2;3]
 *)
 
 let take_front_opt d =
-  if is_empty d then None
-  else if Stdlib.(==) d.cur.prev d.cur
-  then (
-    (* only one cell *)
-    decr_size_ d;
-    Some (take_front_node_ d.cur)
-  ) else (
-    decr_size_ d;
-    let x = take_front_node_ d.cur in
-    if is_zero_ d.cur then (
-      d.cur.prev.next <- d.cur.next;
-      d.cur.next.prev <- d.cur.prev;
-      d.cur <- d.cur.next;
-    );
-    Some x
-  )
+  match d.cur with
+    | Empty -> None
+    | Node cur ->
+      if Stdlib.(==) cur.prev cur
+      then (
+        (* only one cell *)
+        decr_size_ d;
+        let is_zero, x = take_front_node_ cur in
+        if is_zero then d.cur <- Empty;
+        Some x
+      ) else (
+        decr_size_ d;
+        let is_zero, x = take_front_node_ cur in
+        if is_zero then (
+          cur.prev.next <- cur.next;
+          cur.next.prev <- cur.prev;
+          d.cur <- Node cur.next;
+        );
+        Some x
+      )
 
 let take_front d = match take_front_opt d with
   | None -> raise Empty
@@ -246,28 +257,30 @@ let remove_front d = ignore (take_front_opt d)
 *)
 
 let update_front d f =
-  match d.cur.cell with
-    | Zero -> ()
-    | One x ->
-      begin match f x with
-        | None -> if Stdlib.(!=) d.cur.prev d.cur then (
-            d.cur.prev.next <- d.cur.next;
-            d.cur.next.prev <- d.cur.prev;
-            d.cur <- d.cur.next;
-          )
-          else d.cur.cell <- Zero
-        | Some x -> d.cur.cell <- One x
-      end
-    | Two (x, y) ->
-      begin match f x with
-        | None -> d.cur.cell <- One (y)
-        | Some x -> d.cur.cell <- Two (x,y)
-      end
-    | Three (x,y,z) ->
-      begin match f x with
-        | None -> d.cur.cell <- Two (y,z)
-        | Some x -> d.cur.cell <- Three (x,y,z)
-      end
+  match d.cur with
+    | Empty -> ()
+    | Node cur ->
+      match cur.cell with
+        | One x ->
+          begin match f x with
+            | None -> if Stdlib.(!=) cur.prev cur then (
+                cur.prev.next <- cur.next;
+                cur.next.prev <- cur.prev;
+                d.cur <- Node cur.next;
+              )
+              else d.cur <- Empty
+            | Some x -> cur.cell <- One x
+          end
+        | Two (x, y) ->
+          begin match f x with
+            | None -> cur.cell <- One (y)
+            | Some x -> cur.cell <- Two (x,y)
+          end
+        | Three (x,y,z) ->
+          begin match f x with
+            | None -> cur.cell <- Two (y,z)
+            | Some x -> cur.cell <- Three (x,y,z)
+          end
 
 (*$T update_front
   let q = of_list [1;2;3;4;5;6;7] in update_front q (fun _ -> None); to_list q = [2;3;4;5;6;7]
@@ -287,25 +300,27 @@ let update_front d f =
 *)
 
 let update_back d f =
-  let n = d.cur.prev in
-  match n.cell with
-    | Zero -> ()
-    | One x ->
-      begin match f x with
-        | None -> if Stdlib.(!=) d.cur.prev d.cur then remove_node_ n
-          else n.cell <- Zero
-        | Some x -> n.cell <- One x
-      end
-    | Two (x, y) ->
-      begin match f y with
-        | None -> n.cell <- One (x)
-        | Some y -> n.cell <- Two (x,y)
-      end
-    | Three (x,y,z) ->
-      begin match f z with
-        | None -> n.cell <- Two (x,y)
-        | Some z -> n.cell <- Three (x,y,z)
-      end
+  match d.cur with
+    | Empty -> ()
+    | Node cur ->
+      let n = cur.prev in
+      match n.cell with
+        | One x ->
+          begin match f x with
+            | None -> if Stdlib.(!=) cur.prev cur then remove_node_ n
+              else d.cur <- Empty
+            | Some x -> n.cell <- One x
+          end
+        | Two (x, y) ->
+          begin match f y with
+            | None -> n.cell <- One (x)
+            | Some y -> n.cell <- Two (x,y)
+          end
+        | Three (x,y,z) ->
+          begin match f z with
+            | None -> n.cell <- Two (x,y)
+            | Some z -> n.cell <- Three (x,y,z)
+          end
 
 (*$T update_back
   let q = of_list [1;2;3;4;5;6;7] in update_back q (fun _ -> None); to_list q = [1;2;3;4;5;6]
@@ -327,14 +342,16 @@ let update_back d f =
 let iter f d =
   let rec iter f ~first n =
     begin match n.cell with
-      | Zero -> ()
       | One x -> f x
       | Two (x,y) -> f x; f y
       | Three (x,y,z) -> f x; f y; f z
     end;
     if n.next != first then iter f ~first n.next
   in
-  iter f ~first:d.cur d.cur
+  match d.cur with
+    | Empty -> ()
+    | Node cur ->
+      iter f ~first:cur cur
 
 (*$T
   let n = ref 0 in iter (fun _ -> incr n) (of_list [1;2;3]); !n = 3
@@ -362,14 +379,16 @@ let append_back ~into q = iter (push_back into) q
 let fold f acc d =
   let rec aux ~first f acc n =
     let acc = match n.cell with
-      | Zero -> acc
       | One x -> f acc x
       | Two (x,y) -> f (f acc x) y
       | Three (x,y,z) -> f (f (f acc x) y) z
     in
     if Stdlib.(==) n.next first then acc else aux ~first f acc n.next
   in
-  aux ~first:d.cur f acc d.cur
+  match d.cur with
+    | Empty -> acc
+    | Node cur ->
+      aux ~first:cur f acc cur
 
 (*$T
   fold (+) 0 (of_list [1;2;3]) = 6
@@ -436,46 +455,46 @@ let to_rev_list q = fold (fun l x -> x::l) [] q
 let to_list q = List.rev (to_rev_list q)
 
 let size_cell_ = function
-  | Zero -> 0
   | One _ -> 1
   | Two _ -> 2
   | Three _ -> 3
 
 (* filter over a cell *)
 let filter_cell_ f = function
-  | Zero -> Zero
-  | One x as c -> if f x then c else Zero
+  | One x as c -> if f x then Some c else None
   | Two (x,y) as c ->
     let fx = f x in
     let fy = f y in
     begin match fx, fy with
-      | true, true -> c
-      | true, false -> One x
-      | false, true -> One y
-      | _ -> Zero
+      | true, true -> Some c
+      | true, false -> Some (One x)
+      | false, true -> Some (One y)
+      | _ -> None
     end
   | Three (x,y,z) as c ->
     let fx = f x in
     let fy = f y in
     let fz = f z in
     begin match fx, fy, fz with
-      | true, true, true -> c
-      | true, true, false -> Two (x,y)
-      | true, false, true -> Two (x,z)
-      | true, false, false -> One x
-      | false, true, true -> Two (y,z)
-      | false, true, false -> One y
-      | false, false, true -> One z
-      | false, false, false -> Zero
+      | true, true, true -> Some c
+      | true, true, false -> Some (Two (x,y))
+      | true, false, true -> Some (Two (x,z))
+      | true, false, false -> Some (One x)
+      | false, true, true -> Some (Two (y,z))
+      | false, true, false -> Some (One y)
+      | false, false, true -> Some (One z)
+      | false, false, false -> None
     end
 
 let filter_in_place (d:_ t) f : unit =
   (* update size, compute new cell *)
   let update_local_ n =
     d.size <- d.size - size_cell_ n.cell;
-    let new_cell = filter_cell_ f n.cell in
-    d.size <- d.size + size_cell_ new_cell;
-    new_cell
+    match filter_cell_ f n.cell with
+      | None -> None
+      |Some n as new_cell->
+        d.size <- d.size + size_cell_ n;
+        new_cell
   in
   let rec loop ~stop_at n : unit =
     if n != stop_at then (
@@ -484,21 +503,43 @@ let filter_in_place (d:_ t) f : unit =
       let new_cell = update_local_ n in
       (* merge into previous cell *)
       begin match n_prev.cell, new_cell with
-        | _, Zero -> remove_node_ n
-        | Zero, _ -> remove_node_ n; n_prev.cell <- new_cell;
-        | Three _, _ -> n.cell <- new_cell
-        | One x, One y -> remove_node_ n; n_prev.cell <- Two (x,y)
-        | One (x), Two (y,z)
-        | Two (x,y), One z -> remove_node_ n; n_prev.cell <- Three (x,y,z)
-        | One x, Three (y,z,w)
-        | Two (x,y), Two (z,w) -> n_prev.cell <- Three (x,y,z); n.cell <- One w
-        | Two (x,y), Three (z,w1,w2) -> n_prev.cell <- Three (x,y,z); n.cell <- Two (w1,w2)
+        | _, None -> remove_node_ n
+        | Three _, Some new_cell -> n.cell <- new_cell
+        | One x, Some (One y) -> remove_node_ n; n_prev.cell <- Two (x,y)
+        | One (x), Some (Two (y,z))
+        | Two (x,y), Some (One z) -> remove_node_ n; n_prev.cell <- Three (x,y,z)
+        | One x, Some (Three (y,z,w))
+        | Two (x,y), Some (Two (z,w)) -> n_prev.cell <- Three (x,y,z); n.cell <- One w
+        | Two (x,y), Some (Three (z,w1,w2)) -> n_prev.cell <- Three (x,y,z); n.cell <- Two (w1,w2)
       end;
       loop ~stop_at n_next;
     );
   in
-  d.cur.cell <- update_local_ d.cur; (* special case for first cell *)
-  loop ~stop_at:d.cur d.cur.next
+  let rec new_first_cell ~stop_at n =
+    if n != stop_at then (
+      match update_local_ n with
+        | None ->
+          new_first_cell ~stop_at n.next
+        | Some c ->
+          n.cell <- c; Some n
+    ) else None
+  in
+  match d.cur with
+    | Empty -> ()
+    | Node cur ->
+      (* special case for first cell *)
+      match update_local_ cur with
+        | None ->
+          begin match new_first_cell ~stop_at:cur cur.next with
+            | None -> d.cur <- Empty
+            | Some n ->
+              cur.prev.next <- n;
+              n.prev <- cur.prev;
+              d.cur <- Node n;
+              loop ~stop_at:n n.next
+          end
+        | Some c -> cur.cell <- c;
+          loop ~stop_at:cur cur.next
 
 (*$R
    let q = of_list [1;2;3;4;5;6] in
@@ -546,22 +587,25 @@ let of_gen g =
   q
 
 let to_gen q =
-  let first = q.cur in
-  let cell = ref q.cur.cell in
-  let cur = ref q.cur in
-  let rec next () = match !cell with
-    | Zero when Stdlib.(==) (!cur).next first -> None
-    | Zero ->
-      (* go to next node *)
-      let n = !cur in
-      cur := n.next;
-      cell := n.next.cell;
-      next ()
-    | One x -> cell := Zero; Some x
-    | Two (x,y) -> cell := One y; Some x
-    | Three (x,y,z) -> cell := Two (y,z); Some x
-  in
-  next
+  match q.cur with
+    | Empty -> (fun () -> None)
+    | Node cur ->
+      let first = cur in
+      let cell = ref (Some cur.cell) in
+      let cur = ref cur in
+      let rec next () = match !cell with
+        | None when Stdlib.(==) (!cur).next first -> None
+        | None ->
+          (* go to next node *)
+          let n = !cur in
+          cur := n.next;
+          cell := Some (n.next.cell);
+          next ()
+        | Some (One x) -> cell := None; Some x
+        | Some (Two (x,y)) -> cell := Some (One y); Some x
+        | Some (Three (x,y,z)) -> cell := Some (Two (y,z)); Some x
+      in
+      next
 
 (*$T
   of_list [1;2;3] |> to_gen |> of_gen |> to_list = [1;2;3]
@@ -627,3 +671,4 @@ let pp pp_x out d =
        pp_x out x
     ) d;
   Format.fprintf out "}@]"
+