diff --git a/AVL.ml b/AVL.ml
new file mode 100644
index 00000000..fb2ee293
--- /dev/null
+++ b/AVL.ml
@@ -0,0 +1,345 @@
+
+(*
+copyright (c) 2013-2014, simon cruanes
+all rights reserved.
+
+redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+redistributions of source code must retain the above copyright notice, this
+list of conditions and the following disclaimer.  redistributions in binary
+form must reproduce the above copyright notice, this list of conditions and the
+following disclaimer in the documentation and/or other materials provided with
+the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*)
+
+(** {1 AVL trees}
+
+See https://en.wikipedia.org/wiki/AVL_tree *)
+
+type ('a,'b) t =
+  | Empty
+  | Node of ('a,'b) t * 'a * 'b * ('a,'b) t * int
+
+type 'a comparator = 'a -> 'a -> int
+
+let empty = Empty
+
+let _height = function
+  | Empty -> 0
+  | Node (_, _, _, _, h) -> h
+
+let _balance l r = _height l - _height r
+
+(* build the tree *)
+let _make l x y r =
+  Node (l, x, y, r, 1 + max (_height l) (_height r))
+
+let singleton k v = _make empty k v empty
+
+(* balance tree [t] *)
+let _rebalance t = match t with
+  | Empty -> t
+  | Node (l, k1, v1, r, _) ->
+      let b = _balance l r in
+      if b = 2
+      then (* left cases: left tree is too deep *)
+        match l with
+        | Empty -> assert false
+        | Node (ll, k2, v2, lr, _) ->
+          if _balance ll lr = -1
+            then (* left-right *)
+              match lr with
+              | Empty -> assert false
+              | Node (lrl, k3, v3, lrr, _) ->
+                  _make
+                    (_make ll k2 v2 lrl)
+                    k3 v3
+                    (_make lrr k1 v1 r)
+            else (* left-left *)
+              _make
+                ll k2 v2
+                (_make lr k1 v1 r)
+      else if b = -2 (* right cases: symetric *)
+      then match r with
+        | Empty -> assert false
+        | Node (rl, k2, v2, rr, _) ->
+          if _balance rl rr = 1
+            then (* right-left *)
+              match rl with
+              | Empty -> assert false
+              | Node (rll, k3, v3, rlr, _) ->
+                  _make
+                    (_make l k1 v1 rll)
+                    k3 v3
+                    (_make rll k2 v2 rlr)
+            else (* right-right *)
+              _make
+                (_make l k1 v1 rl)
+                k2 v2 rr
+      else t
+
+let _make_balance l k v r =
+  _rebalance (_make l k v r)
+
+let rec fold f acc t = match t with
+  | Empty -> acc
+  | Node (l, x, y, r, _) ->
+      let acc = fold f acc l in
+      let acc = f acc x y in
+      fold f acc r
+
+let rec for_all p t = match t with
+  | Empty -> true
+  | Node (l, x, y, r, _) ->
+      p x y && for_all p l && for_all p r
+
+let rec exists p t = match t with
+  | Empty -> false
+  | Node (l, x, y, r, _) ->
+      p x y || exists p l || exists p r
+
+let rec insert ~cmp t k v = match t with
+  | Empty -> _make empty k v empty
+  | Node (l, k1, v1, r, _) ->
+      let c = cmp k k1 in
+      if c < 0
+        then _make_balance (insert ~cmp l k v) k1 v1 r
+      else if c = 0
+        then _make l k v r
+      else _make_balance l k1 v1 (insert ~cmp r k v)
+
+(* remove the maximal value in the given tree (the only which only has a left
+  child), and return its key/value pair *)
+let rec _remove_max t = match t with
+  | Empty -> assert false
+  | Node (l, k, v, Empty, _) ->
+      l, k, v
+  | Node (l, k, v, r, _) ->
+      let r', k', v' = _remove_max r in
+      _make_balance l k v r', k', v'
+
+exception NoSuchElement
+
+let remove ~cmp t key =
+  let rec _remove t = match t with
+    | Empty -> raise NoSuchElement
+    | Node (l, k, v, r, _) ->
+        let c = cmp key k in
+        if c < 0
+          then _make_balance (_remove l) k v r
+        else if c > 0
+          then _make_balance l k v (_remove r)
+        else
+          (* interesting case: the node to remove is this one. We need
+            to find a replacing node, unless [l] is empty  *)
+          match l with
+          | Empty -> r
+          | Node _ ->
+              let l', k', v' = _remove_max l in
+              _make_balance l' k' v' r
+  in
+  try _remove t
+  with NoSuchElement -> t  (* element not found *)
+
+let update ~cmp t key f = failwith "update: not implemented"
+
+let rec find_exn ~cmp t key = match t with
+  | Empty -> raise Not_found
+  | Node (l, k, v, r, _) ->
+      let c = cmp key k in
+      if c < 0 then find_exn ~cmp l key
+      else if c > 0 then find_exn ~cmp r key
+      else v
+
+let find ~cmp t key =
+  try Some (find_exn ~cmp t key)
+  with Not_found -> None
+
+(* add k,v as strictly maximal element to t. [t] must not contain
+  any key >= k *)
+let rec _add_max k v t = match t with
+  | Empty -> singleton k v
+  | Node (l, k', v', r, _) ->
+      _make_balance l k' v' (_add_max k v r)
+and
+(* same for minimal value *)
+_add_min k v t = match t with
+  | Empty -> singleton k v
+  | Node (l, k', v', r, _) ->
+      _make_balance (_add_min k v l) k' v' r
+
+(* same as [_make] but doesn't assume anything about balance *)
+let rec _join l k v r =
+  match l, r with
+  | Empty, _ -> _add_min k v r
+  | _, Empty -> _add_max k v l
+  | Node (ll, k1, v1, lr, h1), Node (rl, k2, v2, rr, h2) ->
+      if h1 + 1 < h2
+        then (* r is much bigger. join l with rl *)
+          _make_balance (_join l k v rl) k2 v2 rr
+      else if h1 > h2 + 1
+        then
+          _make_balance ll k1 v1 (_join lr k v r)
+      else (* balance uneeded *)
+        _make l k v r
+
+(* concat t1 and t2, where all keys of [t1] are smaller than
+  those of [t2] *)
+let _concat t1 t2 = match t1, t2 with
+  | Empty, t
+  | t, Empty -> t
+  | _ ->
+      let t1', k, v = _remove_max t1 in
+      _join t1' k v t2
+
+let rec split ~cmp t key = match t with
+  | Empty -> empty, None, empty
+  | Node (l, k, v, r, _) ->
+      let c = cmp key k in
+      if c < 0
+        then
+          let ll, result, lr = split ~cmp l key in
+          ll, result, _join lr k v r
+      else if c > 0
+        then
+          let rl, result, rr = split ~cmp r key in
+          _join l k v rl, result, rr
+      else
+        l, Some v, r
+
+(* if k = Some v, join l k v r, else concat l v *)
+let _concat_or_join l k result r = match result with
+  | None -> _concat l r
+  | Some v -> _join l k v r
+
+let rec merge ~cmp f t1 t2 = match t1, t2 with
+  | Empty, Empty -> empty
+  | Node (l1, k1, v1, r1, h1), _ when h1 >= _height t2 ->
+      let l2, result2, r2 = split ~cmp t2 k1 in
+      let result = f (Some k1) result2 in
+      let l = merge ~cmp f l1 l2 in
+      let r = merge ~cmp f r1 r2 in
+      _concat_or_join l k1 result r
+  | _, Node (l2, k2, v2, r2, _) ->
+      let l1, result1, r1 = split ~cmp t1 k2 in
+      let result = f result1 (Some k2) in
+      let l = merge ~cmp f l1 l2 in
+      let r = merge ~cmp f r1 r2 in
+      _concat_or_join l k2 result r
+  | _, Empty -> assert false  (* h1 < heigth h2?? *)
+
+(* invariant: balanced *)
+let rec invariant_balance t = match t with
+  | Empty -> true
+  | Node (l, _, _, r, _) ->
+      abs (_balance l r) < 2
+      && invariant_balance l && invariant_balance r
+
+(* invariant: search tree *)
+let rec invariant_search ~cmp t = match t with
+  | Empty -> true
+  | Node (l, x, _, r, _) ->
+      invariant_search ~cmp l &&
+      invariant_search ~cmp r &&
+      for_all (fun x' _ -> cmp x' x < 0) l &&
+      for_all (fun x' _ -> cmp x' x > 0) r
+
+let of_list ~cmp l =
+  List.fold_left (fun acc (x,y) -> insert ~cmp acc x y) empty l
+
+let to_list t =
+  let rec aux acc t = match t with
+  | Empty -> acc
+  | Node (l, k, v, r, _) ->
+      let acc = aux acc r in
+      let acc = (k,v)::acc in
+      aux acc l
+  in aux [] t
+
+(** {2 Iterators} *)
+
+module type ITERATOR = sig
+  type 'a iter
+
+  val after : cmp:'a comparator -> ('a,'b) t -> 'a -> ('a * 'b) iter
+  val before : cmp:'a comparator -> ('a,'b) t -> 'a -> ('a * 'b) iter
+  val iter : ('a,'b) t -> ('a * 'b) iter
+  val add : cmp:'a comparator -> ('a,'b) t -> ('a * 'b) iter -> ('a,'b) t
+end
+
+type ('a,'b) explore =
+  | Yield of 'a * 'b
+  | Explore of ('a, 'b) t
+
+exception EndOfIter
+
+(* push the tree [t] on the stack [l] *)
+let _push t l = match t with
+  | Empty -> l
+  | Node _ -> Explore t :: l
+
+let _yield k v l = Yield (k,v) :: l
+
+let _has_next = function
+  | [] -> false
+  | _::_ -> true
+
+(* next key,value to yield *)
+let rec _pop l = match l with
+  | [] -> raise EndOfIter
+  | (Yield (k,v))::l' -> k, v, l'
+  | (Explore Empty) :: _ -> assert false
+  | (Explore Node(l, k, v, r, _)::l') ->
+      _pop (_push l (_yield k v (_push r l')))
+
+(* return the intial stack of trees to explore, that
+  are all "after" key (included) *)
+let rec _after ~cmp t key =
+
+module KList = struct
+  type 'a t = [ `Nil | `Cons of 'a * (unit -> 'a t) ]
+
+  let rec _next l () = match l with
+    | [] -> `Nil
+    | _::_ ->
+        let k, v, l' = _pop l in
+        `Cons ((k,v), _next l')
+
+  let iter t = _next (_push t [])
+
+  let rec add ~cmp t (l:'a t) = match l with
+    | `Nil -> t
+    | `Cons ((k,v), l') ->
+        add ~cmp (insert ~cmp t k v) (l' ())
+
+  let after ~cmp t key =
+    (* auxiliary function. We carry a stack [l] of things to do *)
+    let rec aux stack t = match t with
+    | Empty -> `Nil
+    | Node (l, k, v, r, _) ->
+        let c = cmp key k in
+        if c < 0 then aux (_yield k v (_push r stack)) l
+        else if c > 0 then aux stack r
+        else _next (_yield k v stack) ()
+    in aux [] t
+
+  let rec 
+end
+
+module Gen = struct
+  type 'a t = unit -> 'a option
+
+  include ITERATOR with type 'a iter := 'a t
+end
diff --git a/AVL.mli b/AVL.mli
new file mode 100644
index 00000000..545d6c62
--- /dev/null
+++ b/AVL.mli
@@ -0,0 +1,101 @@
+
+(*
+copyright (c) 2013-2014, simon cruanes
+all rights reserved.
+
+redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+redistributions of source code must retain the above copyright notice, this
+list of conditions and the following disclaimer.  redistributions in binary
+form must reproduce the above copyright notice, this list of conditions and the
+following disclaimer in the documentation and/or other materials provided with
+the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*)
+
+(** {1 AVL trees} *)
+
+type ('a,'b) t = private
+  | Empty
+  | Node of ('a,'b) t * 'a * 'b * ('a,'b) t * int
+
+type 'a comparator = 'a -> 'a -> int
+
+val empty : ('a,'b) t
+(** Empty tree *)
+
+val singleton : 'a -> 'b -> ('a,'b) t
+(** Tree with a single node *)
+
+val fold : ('c -> 'a -> 'b -> 'c) -> 'c -> ('a,'b) t -> 'c
+(** Fold on all key/value pairs in the tree *)
+
+val for_all : ('a -> 'b -> bool) -> ('a,'b) t -> bool
+val exists : ('a -> 'b -> bool) -> ('a,'b) t -> bool
+
+val find : cmp:'a comparator -> ('a,'b) t -> 'a -> 'b option
+(** Find the value associated to the key, if any *)
+
+val find_exn : cmp:'a comparator -> ('a,'b) t -> 'a -> 'b
+(** @raise Not_found if the key is not present *)
+
+val insert : cmp:'a comparator -> ('a,'b) t -> 'a -> 'b -> ('a,'b) t
+(** Insertion in the tree *)
+
+val remove : cmp:'a comparator -> ('a,'b) t -> 'a -> ('a,'b) t
+(** Removal from the tree *)
+
+val update : cmp:'a comparator -> ('a,'b) t -> 'a ->
+            ('b option -> ('a * 'b) option) -> ('a,'b) t
+(** Update of the given key binding (subsumes [insert] and [remove]) *)
+
+val split : cmp:'a comparator -> ('a,'b) t -> 'a ->
+            ('a,'b) t * 'b option * ('a,'b) t
+(** [split ~cmp t k] splits [t] into a left part that
+    is smaller than [k], the possible binding of [k],
+    and a part bigger than [k]. *)
+
+val merge : cmp:'a comparator -> 
+            ('a -> 'b option -> 'b option -> 'b option) ->
+            ('a,'b) t -> ('a,'b) t -> ('a,'b) t
+(** Merge two trees together, with the given function *)
+
+val of_list : cmp:'a comparator -> ('a * 'b) list -> ('a,'b) t
+(** Add a list of bindings *)
+
+val to_list : ('a,'b) t -> ('a * 'b) list
+(** List of bindings, in infix order *)
+
+(** {2 Iterators} *)
+
+module type ITERATOR = sig
+  type 'a iter
+
+  val after : cmp:'a comparator -> ('a,'b) t -> 'a -> ('a * 'b) iter
+  val before : cmp:'a comparator -> ('a,'b) t -> 'a -> ('a * 'b) iter
+  val iter : ('a,'b) t -> ('a * 'b) iter
+  val add : cmp:'a comparator -> ('a,'b) t -> ('a * 'b) iter -> ('a,'b) t
+end
+
+module KList : sig
+  type 'a t = [ `Nil | `Cons of 'a * (unit -> 'a t) ]
+
+  include ITERATOR with type 'a iter := 'a t
+end
+
+module Gen : sig
+  type 'a t = unit -> 'a option
+
+  include ITERATOR with type 'a iter := 'a t
+end
diff --git a/_oasis b/_oasis
index 5b77c17c..02861cff 100644
--- a/_oasis
+++ b/_oasis
@@ -41,7 +41,7 @@ Library "containers"
                     UnionFind, SmallSet, Leftistheap, AbsSet, CSM, MultiMap,
                     ActionMan, BV, QCheck, BencodeOnDisk, Show, TTree,
                     HGraph, Automaton, Conv, Levenshtein, Bidir, Iteratee,
-                    Ty, Tell, BencodeStream, RatTerm, Cause, KMP, CircList
+                    Ty, Tell, BencodeStream, RatTerm, Cause, KMP, CircList, AVL
   BuildDepends:     unix
 
 Library "containers_thread"