diff --git a/src/data/CCFun_vec.ml b/src/data/CCFun_vec.ml
new file mode 100644
index 00000000..b91370d4
--- /dev/null
+++ b/src/data/CCFun_vec.ml
@@ -0,0 +1,636 @@
+(* This file is free software, part of containers. See file "license" for more details. *)
+
+(*$inject
+
+  let _listuniq =
+    let g = Q.(small_list (pair small_int small_int)) in
+    Q.map_same_type
+      (fun l ->
+        CCList.sort_uniq ~cmp:(fun a b -> Pervasives.compare (fst a)(fst b)) l
+      ) g
+  ;;
+*)
+
+(** {1 Hash Tries} *)
+
+type 'a sequence = ('a -> unit) -> unit
+type 'a gen = unit -> 'a option
+type 'a printer = Format.formatter -> 'a -> unit
+type 'a ktree = unit -> [`Nil | `Node of 'a * 'a ktree list]
+
+(** {2 Transient IDs} *)
+module Transient = struct
+  type state = { mutable frozen: bool }
+  type t = Nil | St of state
+  let empty = Nil
+  let equal a b = Pervasives.(==) a b
+  let create () = St {frozen=false}
+  let active = function Nil -> false | St st -> not st.frozen
+  let frozen = function Nil -> true | St st -> st.frozen
+  let freeze = function Nil -> () | St st -> st.frozen <- true
+  let with_ f =
+    let r = create() in
+    try
+      let x = f r in
+      freeze r;
+      x
+    with e ->
+      freeze r;
+      raise e
+  exception Frozen
+end
+
+(* function array *)
+module A = struct
+  type 'a t = {
+    arr: 'a array;
+    id: Transient.t;
+  }
+
+  let length_log = 5
+  let max_length = 32
+  let mask = max_length-1
+
+  let () = assert (max_length = 1 lsl length_log)
+
+  let length a = Array.length a.arr
+
+  let create ~id = { arr= [| |]; id; }
+
+  let empty = {arr=[| |]; id=Transient.empty}
+  let is_empty a = length a = 0
+
+  let return x = { arr=[| x |]; id=Transient.empty}
+
+  let owns ~id a =
+    Transient.active id && Transient.equal id a.id
+
+  let get a i =
+    if i<0 || i >= length a then invalid_arg "A.get";
+    Array.unsafe_get a.arr i
+
+  (* push at the back *)
+  let push x a =
+    let n = length a in
+    if n = max_length then invalid_arg "A.push";
+    let arr = Array.make (n+1) x in
+    Array.blit a.arr 0 arr 0 n;
+    {a with arr;}
+
+  let pop a =
+    let n = length a in
+    if n=0 then invalid_arg "A.pop";
+    let arr = Array.sub a.arr 0 (n-1) in
+    {a with arr}
+
+  let append a b =
+    let n_a = length a in
+    let n_b = length b in
+    if n_a + n_b > max_length then invalid_arg "A.append";
+    if n_a = 0 then b
+    else if n_b = 0 then a
+    else (
+      let arr = Array.make (n_a+n_b) (a.arr.(0)) in
+      Array.blit a.arr 0 arr 0 n_a;
+      Array.blit b.arr 0 arr n_a n_b;
+      {id=Transient.empty; arr}
+    )
+
+  let set ~mut a i x =
+    if i<0 || i > length a || i >= max_length then invalid_arg "A.set";
+    if i=length a then (
+      (* insert in a longer copy *)
+      let arr = Array.make (i+1) x in
+      Array.blit a.arr 0 arr 0 i;
+      {a with arr}
+    ) else if mut then (
+      (* replace element at [i] in place *)
+      a.arr.(i) <- x;
+      a
+    ) else (
+      (* replace element at [i] in copy *)
+      let arr = Array.copy a.arr in
+      arr.(i) <- x;
+      {a with arr}
+    )
+
+  let iteri f a = Array.iteri f a.arr
+
+  let fold f acc a = Array.fold_left f acc a.arr
+end
+
+(** {2 Functors} *)
+
+type 'a t = {
+  size: int;
+  leaves: 'a A.t;
+  subs: 'a t A.t;
+}
+(* invariant:
+   - [A.length leaves < A.max_length ==> A.is_empty subs]
+   - either:
+    * [exists n. forall i. subs[i].size = n] (all subtrees of same size)
+    * [exists n i.
+        (forall j<i. sub[j].size=32^{n+1}-1) &
+        (forall j>=i, sub[j].size<32^{n+1}-1)]
+    (prefix of subs has size of complete binary tree; suffix has
+     smaller size (actually decreasing))
+  *)
+
+
+let empty = {size=0; leaves=A.empty; subs=A.empty}
+
+let is_empty {size;_} = size=0
+
+(*$T
+  is_empty empty
+*)
+
+let length {size;_} = size
+
+(*$T
+  not (is_empty (return 2))
+  length (return 2) = 1
+*)
+
+let return x = {leaves=A.return x; subs=A.empty; size=1}
+
+type idx_l =
+  | L1 of int
+  | L2 of int * int
+  | L3 of int * int * int
+  | L4 of int * int * int * int
+  | L_cons of int * idx_l
+
+let cons_idx x1 l = match l with
+  | L1 x2 -> L2 (x1,x2)
+  | L2 (x2,x3) -> L3 (x1,x2,x3)
+  | L3 (x2,x3,x4) -> L4 (x1,x2,x3,x4)
+  | L4 _ | L_cons _ -> L_cons (x1, l)
+
+(* split an index into a low and high parts *)
+let low_idx_ i = i land A.mask
+
+let high_idx_ i = i lsr A.length_log
+
+let combine_idx i j = (i lsl A.length_log) lor j
+
+(* split an index into a high part, < 32, and a low part *)
+let split_idx i : idx_l =
+  let rec aux high low =
+    if high = 0 then low
+    else if high < A.max_length then cons_idx (high-1) low
+    else aux (high_idx_ high) (cons_idx (low_idx_ high) low)
+  in
+  aux (high_idx_ i) (L1 (low_idx_ i))
+
+let get_ (i:int) (m:'a t) : 'a =
+  let rec aux l m = match l with
+    | L1 x1 ->
+      assert (x1 < A.length m.leaves);
+      A.get m.leaves x1
+    | L2 (x1,x2) -> aux (L1 x2) (A.get m.subs x1)
+    | L3 (x1,x2,x3) -> aux (L2 (x2,x3)) (A.get m.subs x1)
+    | L4 (x1,x2,x3,x4) -> aux (L3 (x2,x3,x4)) (A.get m.subs x1)
+    | L_cons (x1,x2) -> aux x2 (A.get m.subs x1)
+  in
+  aux (split_idx i) m
+
+(*$Q
+   _listuniq (fun l -> \
+    let m = of_list l in \
+    List.for_all (fun (i,y) -> get_exn i m = y) @@ List.mapi CCPair.make l)
+*)
+
+let get_exn i v =
+  if i >= 0 && i < length v then get_ i v else raise Not_found
+
+let get i v =
+  if i >= 0 && i < length v then Some (get_ i v) else None
+
+let push_ (i:int) (x:'a) (m:'a t) : 'a t =
+  let rec aux l m = match l with
+    | L1 x1 ->
+      assert (x1=A.length m.leaves);
+      assert (A.length m.leaves < A.max_length);
+      assert (A.is_empty m.subs);
+      {m with size=m.size+1; leaves=A.push x m.leaves}
+    | L2 (x1,x2) -> aux_replace_sub (L1 x2) m x1
+    | L3 (x1,x2,x3) -> aux_replace_sub (L2 (x2,x3)) m x1
+    | L4 (x1,x2,x3,x4) -> aux_replace_sub (L3 (x2,x3,x4)) m x1
+    | L_cons (x1,x2) -> aux_replace_sub x2 m x1
+  and aux_replace_sub l m x =
+    assert (x <= A.length m.subs);
+    (* insert in subtree, possibly a new one *)
+    let sub_m =
+      if x < A.length m.subs then A.get m.subs x else empty
+    in
+    let sub_m = aux l sub_m in
+    {m with size=m.size+1; subs=A.set ~mut:false m.subs x sub_m}
+  in
+  aux (split_idx i) m
+
+let push x (v:_ t) : _ t = push_ v.size x v
+
+let pop_ i (m:'a t) : 'a * 'a t =
+  let rec aux l m = match l with
+    | L1 x1 ->
+      assert (x1+1 = A.length m.leaves); (* last one *)
+      let x = A.get m.leaves x1 in
+      x, {m with size=m.size-1; leaves=A.pop m.leaves}
+    | L2 (x1,x2) -> aux_remove_sub (L1 x2) m x1
+    | L3 (x1,x2,x3) -> aux_remove_sub (L2 (x2,x3)) m x1
+    | L4 (x1,x2,x3,x4) -> aux_remove_sub (L3 (x2,x3,x4)) m x1
+    | L_cons (x1,x2) -> aux_remove_sub x2 m x1
+  and aux_remove_sub l m x =
+    let sub = A.get m.subs x in
+    let y, sub' = aux l sub in
+    if is_empty sub' then (
+      assert (i+1 = A.length m.subs); (* last one *)
+      y, {m with size=m.size-1; subs=A.pop m.subs}
+    ) else (
+      y, {m with size=m.size-1; subs=A.set ~mut:false m.subs x sub}
+    )
+  in
+  aux (split_idx i) m
+
+let pop_exn (v:'a t) : 'a * 'a t =
+  if v.size=0 then failwith "Fun_vec.pop_exn";
+  pop_ v.size v
+
+let pop (v:'a t) : ('a * 'a t) option =
+  if v.size=0 then None else Some (pop_ v.size v)
+
+let iteri ~f (m : 'a t) : unit =
+  (* basically, a 32-way BFS traversal.
+     The queue contains subtrees to explore, along with their high_idx_ offsets *)
+  let q : (int * 'a t) Queue.t = Queue.create() in
+  Queue.push (0,m) q;
+  while not (Queue.is_empty q) do
+    let high, m = Queue.pop q in
+    A.iteri (fun i x -> f (combine_idx high i) x) m.leaves;
+    A.iteri (fun i sub -> Queue.push (combine_idx i high, sub) q) m.subs;
+  done
+
+let foldi ~f ~x m =
+  let acc = ref x in
+  iteri m
+    ~f:(fun i x -> acc := f !acc i x);
+  !acc
+
+let iter ~f m = iteri ~f:(fun _ x -> f x) m
+
+let fold ~f ~x m = foldi ~f:(fun acc _ x -> f acc x) ~x m
+
+let of_list l = List.fold_left (fun v x -> push x v) empty l
+
+let to_list m = fold m ~f:(fun acc x -> x::acc) ~x:[] |> List.rev
+
+(* TODO
+
+(* add [k,v] to the list [l], removing old binding if any *)
+let rec add_list_ k v l = match l with
+  | Nil -> One (k,v)
+  | One (k1, v1) ->
+    if Key.equal k k1 then One (k, v) else Two (k,v,k1,v1)
+  | Two (k1, v1, k2, v2) ->
+    if Key.equal k k1 then Two (k, v, k2, v2)
+    else if Key.equal k k2 then Two (k, v, k1, v1)
+    else Cons (k, v, l)
+  | Cons (k', v', tail) ->
+    if Key.equal k k'
+    then Cons (k, v, tail) (* replace *)
+    else Cons (k', v', add_list_ k v tail)
+
+let node_ leaf a = N (leaf, a)
+
+
+(* [h]: hash, with the part required to reach this leaf removed
+    [id] is the transient ID used for mutability *)
+let rec add_ ~id k v ~h m = match m with
+  | E -> S (h, k, v)
+  | S (h', k', v') ->
+    if Hash.equal h h'
+    then if Key.equal k k'
+      then S (h, k, v)  (* replace *)
+      else L (h, Cons (k, v, Cons (k', v', Nil)))
+    else
+      make_array_ ~id ~leaf:(Cons (k', v', Nil)) ~h_leaf:h' k v ~h
+  | L (h', l) ->
+    if Hash.equal h h'
+    then L (h, add_list_ k v l)
+    else (* split into N *)
+      make_array_ ~id ~leaf:l ~h_leaf:h' k v ~h
+  | N (leaf, a) ->
+    if Hash.is_0 h
+    then node_ (add_list_ k v leaf) a
+    else
+      let mut = A.owns ~id a in (* can we modify [a] in place? *)
+      node_ leaf (add_to_array_ ~id ~mut k v ~h a)
+
+(* make an array containing a leaf, and insert (k,v) in it *)
+and make_array_ ~id ~leaf ~h_leaf:h' k v ~h =
+  let a = A.create ~id in
+  let a, leaf =
+    if Hash.is_0 h' then a, leaf
+    else
+      (* put leaf in the right bucket *)
+      let i = Hash.rem h' in
+      let h'' = Hash.quotient h' in
+      A.set ~mut:true a i (L (h'', leaf)), Nil
+  in
+  (* then add new node *)
+  let a, leaf =
+    if Hash.is_0 h then a, add_list_ k v leaf
+    else add_to_array_ ~id ~mut:true k v ~h a, leaf
+  in
+  N (leaf, a)
+
+(* add k->v to [a] *)
+and add_to_array_ ~id ~mut k v ~h a =
+  (* insert in a bucket *)
+  let i = Hash.rem h in
+  let h' = Hash.quotient h in
+  A.update ~default:E ~mut a i (fun x -> add_ ~id k v ~h:h' x)
+
+let add k v m = add_ ~id:Transient.empty k v ~h:(hash_ k) m
+
+(*$Q
+   _listuniq (fun l -> \
+      let m = List.fold_left (fun m (x,y) -> add x y m) empty l in \
+      List.for_all (fun (x,y) -> get_exn x m = y) l)
+*)
+
+let add_mut ~id k v m =
+  if Transient.frozen id then raise Transient.Frozen;
+  add_ ~id k v ~h:(hash_ k) m
+
+(*$R
+  let lsort = List.sort Pervasives.compare in
+  let m = of_list [1, 1; 2, 2] in
+  let id = Transient.create() in
+  let m' = add_mut ~id 3 3 m in
+  let m' = add_mut ~id 4 4 m' in
+  assert_equal [1, 1; 2, 2] (to_list m |> lsort);
+  assert_equal [1, 1; 2, 2; 3,3; 4,4] (to_list m' |> lsort);
+  Transient.freeze id;
+  assert_bool "must raise"
+    (try ignore(add_mut ~id 5 5 m'); false with Transient.Frozen -> true)
+*)
+
+
+exception LocalExit
+
+let is_empty_arr_ a =
+  try
+    A.iter (fun t -> if not (is_empty t) then raise LocalExit) a;
+    true
+  with LocalExit -> false
+
+let is_empty_list_ = function
+  | Nil -> true
+  | One _
+  | Two _
+  | Cons _ -> false
+
+let rec remove_list_ k l = match l with
+  | Nil -> Nil
+  | One (k', _) ->
+    if Key.equal k k' then Nil else l
+  | Two (k1, v1, k2, v2) ->
+    if Key.equal k k1 then One (k2, v2)
+    else if Key.equal k k2 then One (k1, v1)
+    else l
+  | Cons (k', v', tail) ->
+    if Key.equal k k'
+    then tail
+    else Cons (k', v', remove_list_ k tail)
+
+let rec remove_rec_ ~id k ~h m = match m with
+  | E -> E
+  | S (_, k', _) ->
+    if Key.equal k k' then E else m
+  | L (h, l) ->
+    let l = remove_list_ k l in
+    if is_empty_list_ l then E else L (h, l)
+  | N (leaf, a) ->
+    let leaf, a =
+      if Hash.is_0 h
+      then remove_list_ k leaf, a
+      else
+        let i = Hash.rem h in
+        let h' = Hash.quotient h in
+        let new_t = remove_rec_ ~id k ~h:h' (A.get ~default:E a i) in
+        if is_empty new_t
+        then leaf, A.remove a i (* remove sub-tree *)
+        else
+          let mut = A.owns ~id a in
+          leaf, A.set ~mut a i new_t
+    in
+    if is_empty_list_ leaf && is_empty_arr_ a
+    then E
+    else N (leaf, a)
+
+let remove k m = remove_rec_ ~id:Transient.empty k ~h:(hash_ k) m
+
+let remove_mut ~id k m =
+  if Transient.frozen id then raise Transient.Frozen;
+  remove_rec_ ~id k ~h:(hash_ k) m
+
+(*$QR
+  _listuniq (fun l ->
+    let m = of_list l in
+    List.for_all
+      (fun (x,_) ->
+        let m' = remove x m in
+        not (mem x m') &&
+        cardinal m' = cardinal m - 1 &&
+        List.for_all
+          (fun (y,v) -> y = x || get_exn y m' = v)
+          l
+    ) l
+  )
+*)
+
+let update_ ~id k f m =
+  let h = hash_ k in
+  let opt_v = try Some (get_ k ~h m) with Not_found -> None in
+  match opt_v, f opt_v with
+    | None, None -> m
+    | Some _, Some v
+    | None, Some v -> add_ ~id k v ~h m
+    | Some _, None -> remove_rec_ ~id k ~h m
+
+let update k ~f m = update_ ~id:Transient.empty k f m
+
+let update_mut ~id k ~f m =
+  if Transient.frozen id then raise Transient.Frozen;
+  update_ ~id k f m
+
+(*$R
+  let m = of_list [1, 1; 2, 2; 5, 5] in
+  let m' = update 4
+    (function
+    | None -> Some 4
+    | Some _ -> Some 0
+    ) m
+  in
+  assert_equal [1,1; 2,2; 4,4; 5,5] (to_list m' |> List.sort Pervasives.compare);
+*)
+
+let iter ~f t =
+  let rec aux = function
+    | E -> ()
+    | S (_, k, v) -> f k v
+    | L (_,l) -> aux_list l
+    | N (l,a) -> aux_list l; A.iter aux a
+  and aux_list = function
+    | Nil -> ()
+    | One (k,v) -> f k v
+    | Two (k1,v1,k2,v2) -> f k1 v1; f k2 v2
+    | Cons (k, v, tl) -> f k v; aux_list tl
+  in
+  aux t
+
+let fold ~f ~x:acc t =
+  let rec aux acc t = match t with
+    | E -> acc
+    | S (_,k,v) -> f acc k v
+    | L (_,l) -> aux_list acc l
+    | N (l,a) -> let acc = aux_list acc l in A.fold aux acc a
+  and aux_list acc l = match l with
+    | Nil -> acc
+    | One (k,v) -> f acc k v
+    | Two (k1,v1,k2,v2) -> f (f acc k1 v1) k2 v2
+    | Cons (k, v, tl) -> let acc = f acc k v in aux_list acc tl
+  in
+  aux acc t
+
+(*$T
+  let l = CCList.(1 -- 10 |> map (fun x->x,x)) in  \
+  of_list l \
+    |> fold ~f:(fun acc x y -> (x,y)::acc) ~x:[] \
+    |> List.sort Pervasives.compare = l
+*)
+
+let cardinal m = fold ~f:(fun n _ _ -> n+1) ~x:0 m
+
+let to_list m = fold ~f:(fun acc k v -> (k,v)::acc) ~x:[] m
+
+let add_list_mut ~id m l =
+  List.fold_left (fun acc (k,v) -> add_mut ~id k v acc) m l
+
+let add_list m l =
+  Transient.with_ (fun id -> add_list_mut ~id m l)
+
+let of_list l = add_list empty l
+
+let add_seq_mut ~id m seq =
+  let m = ref m in
+  seq (fun (k,v) -> m := add_mut ~id k v !m);
+  !m
+
+let add_seq m seq =
+  Transient.with_ (fun id -> add_seq_mut ~id m seq)
+
+let of_seq s = add_seq empty s
+
+let to_seq m yield = iter ~f:(fun k v -> yield (k,v)) m
+
+(*$Q
+  _listuniq (fun l -> \
+    (List.sort Pervasives.compare l) = \
+      (l |> Sequence.of_list |> of_seq |> to_seq |> Sequence.to_list \
+        |> List.sort Pervasives.compare) )
+*)
+
+let rec add_gen_mut ~id m g = match g() with
+  | None -> m
+  | Some (k,v) -> add_gen_mut ~id (add_mut ~id k v m) g
+
+let add_gen m g =
+  Transient.with_ (fun id -> add_gen_mut ~id m g)
+
+let of_gen g = add_gen empty g
+
+(* traverse the tree by increasing hash order, where the order compares
+   hashes lexicographically by A.length_log-wide chunks of bits,
+   least-significant chunks first *)
+let to_gen m =
+  let st = Stack.create() in
+  Stack.push m st;
+  let rec next() =
+    if Stack.is_empty st then None
+    else match Stack.pop st with
+      | E -> next ()
+      | S (_,k,v) -> Some (k,v)
+      | L (_, Nil) -> next()
+      | L (_, One (k,v)) -> Some (k,v)
+      | L (h, Two (k1,v1,k2,v2)) ->
+        Stack.push (L (h, One (k2,v2))) st;
+        Some (k1,v1)
+      | L (h, Cons(k,v,tl)) ->
+        Stack.push (L (h, tl)) st;  (* tail *)
+        Some (k,v)
+      | N (l, a) ->
+        A.iter
+          (fun sub -> Stack.push sub st)
+          a;
+        Stack.push (L (Hash.zero, l)) st;  (* leaf *)
+        next()
+  in
+  next
+
+(*$Q
+  _listuniq (fun l -> \
+    (List.sort Pervasives.compare l) = \
+      (l |> Gen.of_list |> of_gen |> to_gen |> Gen.to_list \
+        |> List.sort Pervasives.compare) )
+*)
+
+let choose m = to_gen m ()
+
+(*$T
+  choose empty = None
+  choose (of_list [1,1; 2,2]) <> None
+*)
+
+let choose_exn m = match choose m with
+  | None -> raise Not_found
+  | Some (k,v) -> k, v
+
+let pp ppk ppv out m =
+  let first = ref true in
+  iter m
+    ~f:(fun k v ->
+      if !first then first := false else Format.fprintf out ";@ ";
+      ppk out k;
+      Format.pp_print_string out " -> ";
+      ppv out v
+    )
+
+let rec as_tree m () = match m with
+  | E -> `Nil
+  | S (h,k,v) -> `Node (`L ((h:>int), [k,v]), [])
+  | L (h,l) -> `Node (`L ((h:>int), list_as_tree_ l), [])
+  | N (l,a) -> `Node (`N, as_tree (L (Hash.zero, l)) :: array_as_tree_ a)
+and list_as_tree_ l = match l with
+  | Nil -> []
+  | One (k,v) -> [k,v]
+  | Two (k1,v1,k2,v2) -> [k1,v1; k2,v2]
+  | Cons (k, v, tail) -> (k,v) :: list_as_tree_ tail
+and array_as_tree_ a = A.fold (fun acc t -> as_tree t :: acc) [] a
+   *)
+
+(*   TODO: $R again?
+  let m = of_list CCList.( (501 -- 1000) @ (500 -- 1) |> map (fun i->i,i)) in
+  assert_equal ~printer:CCInt.to_string 1000 (length m);
+  assert_bool "check all get"
+    (Sequence.for_all (fun i -> i = get_exn i m) Sequence.(1 -- 1000));
+  let m = Sequence.(501 -- 1000 |> fold (fun m i -> remove i m) m) in
+  assert_equal ~printer:CCInt.to_string 500 (length m);
+  assert_bool "check all get after remove"
+    (Sequence.for_all (fun i -> i = get_exn i m) Sequence.(1 -- 500));
+  assert_bool "check all get after remove"
+    (Sequence.for_all (fun i -> None = get i m) Sequence.(501 -- 1000));
+*)
diff --git a/src/data/CCFun_vec.mli b/src/data/CCFun_vec.mli
new file mode 100644
index 00000000..ad2a3d88
--- /dev/null
+++ b/src/data/CCFun_vec.mli
@@ -0,0 +1,142 @@
+(* This file is free software, part of containers. See file "license" for more details. *)
+
+(** {1 Functional Vectors} *)
+
+(** Tree with a large branching factor for logarithmic operations with
+    a low multiplicative factor.
+
+    {b status: unstable}
+
+    @since NEXT_RELEASE
+*)
+
+type 'a sequence = ('a -> unit) -> unit
+type 'a gen = unit -> 'a option
+type 'a printer = Format.formatter -> 'a -> unit
+type 'a ktree = unit -> [`Nil | `Node of 'a * 'a ktree list]
+
+(** {2 Transient Identifiers} *)
+module Transient : sig
+  type t
+  (** Identifiers for transient modifications. A transient modification
+      is uniquely identified by a [Transient.t]. Once [Transient.freeze r]
+      is called, [r] cannot be used to modify the structure again. *)
+
+  val create : unit -> t
+  (** Create a new, active ID. *)
+
+  val equal : t -> t -> bool
+  (** Equality between IDs. *)
+
+  val frozen : t -> bool
+  (** [frozen i] returns [true] if [freeze i] was called before. In this case,
+      the ID cannot be used for modifications again. *)
+
+  val active : t -> bool
+  (** [active i] is [not (frozen i)]. *)
+
+  val freeze : t -> unit
+  (** [freeze i] makes [i] unusable for new modifications. The values
+      created with [i] will now be immutable. *)
+
+  val with_ : (t -> 'a) -> 'a
+  (** [with_ f] creates a transient ID [i], calls [f i],
+      freezes the ID [i] and returns the result of [f i]. *)
+
+  exception Frozen
+  (** Raised when a frozen ID is used. *)
+end
+
+(** {2 Signature} *)
+
+type 'a t
+
+val empty : 'a t
+
+val is_empty : _ t -> bool
+
+val return : 'a -> 'a t
+
+val length : _ t -> int
+
+val push : 'a -> 'a t -> 'a t
+(** Add element at the end *)
+
+val get : int -> 'a t -> 'a option
+
+val get_exn : int -> 'a t -> 'a
+(** @raise Not_found if key not present. *)
+
+val pop_exn : 'a t -> 'a * 'a t
+(** Pop last element *)
+
+val iter : f:('a -> unit) -> 'a t -> unit
+
+val iteri : f:(int -> 'a -> unit) -> 'a t -> unit
+
+val fold : f:('b -> 'a -> 'b) -> x:'b -> 'a t -> 'b
+
+val foldi : f:('b -> int -> 'a -> 'b) -> x:'b -> 'a t -> 'b
+
+(* TODO
+
+val push_mut : id:Transient.t -> 'a -> 'a t -> 'a t
+(** [add_mut ~id k v m] behaves like [add k v m], except it will mutate
+    in place whenever possible. Changes done with an [id] might affect all
+    versions of the structure obtained with the same [id] (but not
+    other versions).
+    @raise Transient.Frozen if [id] is frozen. *)
+
+val pop_mut : id:Transient.t -> 'a t -> 'a * 'a t
+(** Same as {!remove}, but modifies in place whenever possible.
+    @raise Transient.Frozen if [id] is frozen. *)
+
+val append : 'a t -> 'a t -> 'a t
+
+val append_mut : id:Transient.t -> into:'a t -> 'a t -> 'a t
+
+val map : ('a -> 'b) -> 'a t -> 'b t
+    *)
+
+(** {6 Conversions} *)
+
+val to_list : 'a t -> 'a list
+
+val of_list : 'a list -> 'a t
+
+(* TODO
+
+val add_list : 'a t -> 'a list -> 'a t
+
+val add_list_mut : id:Transient.t -> 'a t -> 'a list -> 'a t
+(** @raise Frozen if the ID is frozen. *)
+
+val add_seq : 'a t -> 'a sequence -> 'a t
+
+val add_seq_mut : id:Transient.t -> 'a t -> 'a sequence -> 'a t
+(** @raise Frozen if the ID is frozen. *)
+
+val of_seq : 'a sequence -> 'a t
+
+val to_seq : 'a t -> 'a sequence
+
+val add_gen : 'a t -> 'a gen -> 'a t
+
+val add_gen_mut : id:Transient.t -> 'a t -> 'a gen -> 'a t
+(** @raise Frozen if the ID is frozen. *)
+
+val of_gen : 'a gen -> 'a t
+
+val to_gen : 'a t -> 'a gen
+
+(** {6 IO} *)
+
+val pp : 'a printer -> 'a t printer
+
+val as_tree : 'a t -> [`L of int * 'a list | `N ] ktree
+(** For debugging purpose: explore the structure of the tree,
+    with [`L (h,l)] being a leaf (with shared hash [h])
+    and [`N] an inner node. *)
+
+   *)
+