diff --git a/_oasis b/_oasis
index df6756cb..e9165789 100644
--- a/_oasis
+++ b/_oasis
@@ -66,7 +66,7 @@ Library "containers_misc"
                     Bij, PiCalculus, Bencode, Sexp, RAL,
                     UnionFind, SmallSet, AbsSet, CSM,
                     ActionMan, BencodeOnDisk, TTree, PrintBox,
-                    HGraph, Automaton, Conv, Bidir, Iteratee,
+                    HGraph, Automaton, Conv, Bidir, Iteratee, BTree,
                     Ty, Tell, BencodeStream, RatTerm, Cause, AVL, ParseReact
   BuildDepends:     unix,containers
   FindlibName:      misc
diff --git a/misc/bTree.ml b/misc/bTree.ml
new file mode 100644
index 00000000..7e0f6218
--- /dev/null
+++ b/misc/bTree.ml
@@ -0,0 +1,374 @@
+
+(*
+copyright (c) 2013, 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 B-Trees} *)
+
+type 'a sequence = ('a -> unit) -> unit
+type 'a ktree = unit -> [`Nil | `Node of 'a * 'a ktree list]
+
+(** {2 signature} *)
+
+module type S = sig
+  type key
+  type 'a t
+
+  val create : unit -> 'a t
+  (** Empty map *)
+
+  val size : _ t -> int
+  (** Number of bindings *)
+
+  val add : key -> 'a -> 'a t -> unit
+  (** Add a binding to the tree. Erases the old binding, if any *)
+
+  val remove : key -> 'a t -> unit
+  (** Remove the given key, or does nothing if the key isn't present *)
+
+  val get : key -> 'a t -> 'a option
+  (** Key lookup *)
+
+  val get_exn : key -> 'a t -> 'a
+  (** Unsafe version of {!get}.
+      @raise Not_found if the key is not present *)
+
+  val fold : ('b -> key -> 'a -> 'b) -> 'b -> 'a t -> 'b
+  (** Fold on bindings *)
+
+  val of_list : (key * 'a) list -> 'a t
+  val to_list : 'a t -> (key * 'a) list
+  val to_tree : 'a t -> (key * 'a) list ktree
+end
+
+(** {2 Functor} *)
+
+module type ORDERED = sig
+  type t
+  val compare : t -> t -> int
+end
+
+module Make(X : ORDERED) = struct
+  type key = X.t
+
+  let _len_node = 1 lsl 6
+  let _min_len = _len_node / 2
+
+  (* B-tree *)
+  type 'a tree =
+    | E
+    | N of 'a node
+    | L of 'a node
+
+  (* an internal node, with children separated by keys/value pairs.
+    the [i]-th key of [n.keys] separates the subtrees [n.children.(i)] and
+    [n.children.(i+1)] *)
+  and 'a node = {
+    keys : key array;
+    values : 'a array;
+    children : 'a tree array; (* with one more slot *)
+    mutable size : int;  (* number of bindings in the [key] *)
+  }
+
+  type 'a t = {
+    mutable root : 'a tree;
+    mutable cardinal : int;
+  }
+
+  let is_empty = function
+    | E -> true
+    | N _
+    | L _ -> false
+
+  let create () = {
+    root=E;
+    cardinal=0;
+  }
+
+  (* build a new leaf with the given binding *)
+  let _make_singleton k v = {
+    keys = Array.make _len_node k;
+    values = Array.make _len_node v;
+    children = Array.make (_len_node+1) E;
+    size = 1;
+  }
+
+  (* slice of [l] starting at indices [i], of length [len] *)
+  let _make_slice l i len =
+    assert (len>0);
+    let k = l.keys.(i) and v = l.values.(i)  in
+    let l' = {
+      keys = Array.make _len_node k;
+      values = Array.make _len_node v;
+      children = Array.make (_len_node+1) E;
+      size = len;
+    } in
+    Array.blit l.keys i l'.keys 0 len;
+    Array.blit l.values i l'.values 0 len;
+    Array.blit l.children (i+1) l'.children 1 (len-1);
+    l'
+
+  let _full_node n = n.size = _len_node
+  let _empty_node n = n.size = 0
+
+  let size t = t.cardinal
+
+  let rec _fold f acc t = match t with
+    | E -> ()
+    | L n ->
+        for i=0 to n.size-1 do
+          acc := f !acc n.keys.(i) n.values.(i)
+        done
+    | N n ->
+        for i=0 to n.size-1 do
+          _fold f acc n.children.(i);
+          acc := f !acc n.keys.(i) n.values.(i);
+        done;
+        acc := f !acc n.keys.(n.size) n.values.(n.size)
+
+  let fold f acc t =
+    let acc = ref acc in
+    _fold f acc t.root;
+    !acc
+
+  type lookup_result =
+    | At of int
+    | After of int
+
+  (* lookup in a node. *)
+  let rec _lookup_rec l k i =
+    if i = l.size then After (i-1)
+    else match X.compare k l.keys.(i) with
+      | 0 -> At i
+      | n when n<0 -> After (i-1)
+      | _ -> _lookup_rec l k (i+1)
+
+  let _lookup l k =
+    if l.size = 0 then After ~-1
+    else _lookup_rec l k 0
+
+  (* recursive lookup in a tree *)
+  let rec _get_exn k t = match t with
+    | E -> raise Not_found
+    | L l ->
+        begin match _lookup l k with
+        | At i -> l.values.(i)
+        | After _ -> raise Not_found
+        end
+    | N n ->
+        assert (n.size > 0);
+        match _lookup n k with
+        | At i -> n.values.(i)
+        | After i -> _get_exn k n.children.(i+1)
+
+  let get_exn k t = _get_exn k t.root
+
+  let get k t =
+    try Some (_get_exn k t.root)
+    with Not_found -> None
+
+  (* sorted insertion into a leaf that has room and doesn't contain the key *)
+  let _insert_sorted l k v i =
+    (* make room by shifting to the right *)
+    let len = l.size - i in
+    Array.blit l.keys i l.keys (i+1) len;
+    Array.blit l.values i l.values (i+1) len;
+    l.keys.(i) <- k;
+    l.values.(i) <- v;
+    l.size <- l.size + 1;
+
+  (* what happens when we insert a value *)
+  type 'a add_result =
+    | NewTree of 'a tree
+    | Add
+    | Replace
+    | Split of 'a tree * key * 'a * 'a tree
+
+  let _add_leaf k v t l =
+    match _lookup l k with
+    | At i ->
+        l.values.(i) <- v;
+        Replace
+    | After i ->
+        if _full_node l
+        then (
+          (* split. [k'] and [v']: separator for split *)
+          let j = _len_node/2 in
+          let k' = l.keys.(j) in
+          let v' = l.values.(j) in
+          let left = _make_slice l 0 j in
+          let right = _make_slice l (j+1) (_len_node-j-1) in
+          (* insert in proper sub-leaf *)
+          (if i<j
+            then _insert_sorted left k v i
+            else _insert_sorted right k v (i-j)
+          );
+          Split (L left, k', v', L right)
+        ) else (
+          (* just insert at sorted position *)
+          _insert_sorted l k v i;
+          Add
+        )
+
+  let _insert_node n i k v sub1 sub2 =
+    assert (not(_full_node n));
+    Array.blit n.keys i n.keys (i+1) (n.size-i);
+    Array.blit n.values i n.values (i+1) (n.size-i);
+    Array.blit n.children (i+1) n.children (i+2) (n.size-i);
+    n.keys.(i) <- k;
+    n.values.(i) <- v;
+    (* erase subtree with sub1,sub2 *)
+    n.children.(i) <- sub1;
+    n.children.(i+1) <- sub2;
+    n.size <- n.size + 1;
+    ()
+
+  (* return a boolean indicating whether the key was already
+    present, and a new tree. *)
+  let rec _add k v t = match t with
+    | E -> NewTree (L (_make_singleton k v))
+    | L l -> _add_leaf k v t l
+    | N n ->
+        match _lookup n k with
+        | At i ->
+            n.values.(i) <- v;
+            Replace
+        | After i ->
+          assert (X.compare n.keys.(i) k < 0);
+          let sub = n.children.(i+1) in
+          match _add k v sub with
+          | NewTree t' ->
+              n.children.(i+1) <- t';
+              Add
+          | Add -> Add
+          | Replace -> Replace
+          | Split (sub1, k', v', sub2) ->
+              assert (X.compare n.keys.(i) k' < 0);
+              if _full_node n
+              then (
+                (* split this node too! *)
+                let j = _len_node/2 in
+                let left = _make_slice n 0 j in
+                let right = _make_slice n (j+1) (_len_node-j-1) in
+                left.children.(0) <- n.children.(0);
+                right.children.(_len_node-j) <- n.children.(_len_node);
+                (* insert k' and subtrees in the correct tree *)
+                (if i<j
+                  then _insert_node left (i+1) k' v' sub1 sub2
+                  else _insert_node right (i+1-j) k' v' sub1 sub2
+                );
+                (* return the split tree *)
+                let k'' = n.keys.(j) in
+                let v'' = n.values.(j) in
+                Split (N left, k'', v'', N right)
+              ) else (
+                (* insertion of [k] at position [i+1] *)
+                _insert_node n (i+1) k' v' sub1 sub2;
+                Add
+              )
+
+  let add k v t =
+    match _add k v t.root with
+      | NewTree t' ->
+          t.cardinal <- t.cardinal + 1;
+          t.root <- t'
+      | Replace -> ()
+      | Add -> t.cardinal <- t.cardinal + 1
+      | Split (sub1, k, v, sub2) ->
+          (* make a new root with one child *)
+          let n = _make_singleton k v in
+          n.children.(0) <- sub1;
+          n.children.(1) <- sub2;
+          t.cardinal <- t.cardinal + 1;
+          t.root <- N n
+
+  let of_list l =
+    let t = create() in
+    List.iter (fun (k, v) -> add k v t) l;
+    t
+
+  let to_list t =
+    List.rev (fold (fun acc k v -> (k,v)::acc) [] t)
+
+  let rec _to_tree t () = match t with
+    | E -> `Nil
+    | L n
+    | N n ->
+        let l = ref [] and children = ref [] in
+        for i=0 to n.size-1 do
+          l := (n.keys.(i),n.values.(i)) :: !l;
+          children := n.children.(i) :: !children
+        done;
+        children := n.children.(n.size) :: !children;
+        children := List.filter (function E -> false | _ -> true) !children;
+        `Node (List.rev !l, List.rev_map _to_tree !children)
+
+  let to_tree t = _to_tree t.root
+
+  (*$T
+    let module T = Make(CCInt) in \
+      let t = T.of_list (CCList.(1--1000) |> List.map (fun x->x, string_of_int x)) in \
+      T.get 1 t = Some "1"
+    let module T = Make(CCInt) in \
+      let t = T.of_list (CCList.(1--1000) |> List.map (fun x->x, string_of_int x)) in \
+      T.get 3 t = Some "3"
+    let module T = Make(CCInt) in \
+      let t = T.of_list (CCList.(1--1000) |> List.map (fun x->x, string_of_int x)) in \
+      T.get 4 t = None
+  *)
+
+  (* remove the key if present.  TODO
+  let rec _remove k t = match t with
+    | E -> false, E
+    | N n ->
+        assert (n.size > 0);
+        if X.compare k (_min_key n) < 0
+        then (
+          let removed, left' = _remove k n.left in
+          n.left <- left';
+          n.depth <- 1+max (_depth n.left) (_depth n.right);
+          removed, _balance t
+        ) else if X.compare k (_max_key n) > 0
+        then (
+          let removed, right' = _remove k n.right in
+          n.right <- right';
+          n.depth <- 1+max (_depth n.left) (_depth n.right);
+          removed, _balance t
+        )
+        else try
+          let i = _lookup n k 0 in
+          if n.size = 1  (* TODO: actually minimal threshold should be higher *)
+          then true, E
+          else (
+            let len = n.size - i in
+            Array.blit n.keys (i+1) n.keys i len;
+            Array.blit n.values (i+1) n.values i len;
+            true, t
+          )
+        with Not_found ->
+          false, t  (* not to be removed *)
+  *)
+
+  let remove k t = assert false (* TODO *)
+end
diff --git a/misc/bTree.mli b/misc/bTree.mli
new file mode 100644
index 00000000..0d068d9c
--- /dev/null
+++ b/misc/bTree.mli
@@ -0,0 +1,90 @@
+
+(*
+copyright (c) 2013, 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 B-Trees}
+
+Shallow, cache-friendly associative data structure.
+See {{: https://en.wikipedia.org/wiki/B-tree} wikipedia}.
+
+Not thread-safe.  *)
+
+type 'a sequence = ('a -> unit) -> unit
+type 'a ktree = unit -> [`Nil | `Node of 'a * 'a ktree list]
+
+(** {2 signature} *)
+
+module type S = sig
+  type key
+  type 'a t
+
+  val create : unit -> 'a t
+  (** Empty map *)
+
+  val size : _ t -> int
+  (** Number of bindings *)
+
+  val add : key -> 'a -> 'a t -> unit
+  (** Add a binding to the tree. Erases the old binding, if any *)
+
+  val remove : key -> 'a t -> unit
+  (** Remove the given key, or does nothing if the key isn't present *)
+
+  val get : key -> 'a t -> 'a option
+  (** Key lookup *)
+
+  val get_exn : key -> 'a t -> 'a
+  (** Unsafe version of {!get}.
+      @raise Not_found if the key is not present *)
+
+  val fold : ('b -> key -> 'a -> 'b) -> 'b -> 'a t -> 'b
+  (** Fold on bindings *)
+
+  val of_list : (key * 'a) list -> 'a t
+  val to_list : 'a t -> (key * 'a) list
+  val to_tree : 'a t -> (key * 'a) list ktree
+end
+
+(** {2 Functor that builds trees for comparable keys} *)
+
+module type ORDERED = sig
+  type t
+  val compare : t -> t -> int
+end
+
+module Make(X : ORDERED) : S with type key = X.t
+
+(* note: to print a B-tree in dot:
+{[
+let t = some_btree in
+let t' = CCKTree.map
+  (fun t ->
+    [`Shape "square";
+     `Label (CCPrint.to_string (CCList.pp (CCPair.pp CCInt.pp CCString.pp)) t)]
+  ) (T.to_tree t);;
+CCPrint.to_file "/tmp/some_file.dot" "%a\n" (CCKTree.Dot.pp_single "btree") t';
+]}
+*)
+