diff --git a/README.md b/README.md
index 910c30ce..1d5b99f9 100644
--- a/README.md
+++ b/README.md
@@ -42,6 +42,7 @@ monadic futures, and MVars (concurrent boxes)
     a functorial interface (replaced by PHashtbl)
 - `Gen` and `Sequence`, generic iterators structures.
 - `UnionFind`, a functorial imperative Union-Find structure.
+- `HGraph`, a structure of generalized hypergraphs
 
 Some serialisation formats are also implemented, with a streaming, non-blocking
 interface that allows the user to feed the input in chunk by chunk (useful
diff --git a/_oasis b/_oasis
index 0b3bbe4a..945d8cb0 100644
--- a/_oasis
+++ b/_oasis
@@ -39,7 +39,8 @@ Library "containers"
                     PHashtbl, Sequence, SkipList, SplayTree, SplayMap, Univ,
                     Vector, Bij, PiCalculus, Bencode, Sexp, RAL, MultiSet,
                     UnionFind, SmallSet, Leftistheap, AbsSet, CSM, MultiMap,
-                    ActionMan, BV, QCheck, BencodeOnDisk, Show, TTree
+                    ActionMan, BV, QCheck, BencodeOnDisk, Show, TTree,
+                    HGraph
   BuildDepends:     unix
 
 Library "containers_thread"
diff --git a/_tags b/_tags
index 3f96a872..ab5e37c9 100644
--- a/_tags
+++ b/_tags
@@ -1,5 +1,5 @@
 # OASIS_START
-# DO NOT EDIT (digest: 727237717e19643a155e5e70e3495f8e)
+# DO NOT EDIT (digest: dcf1c0cacacb67335a863b18858eaeb7)
 # Ignore VCS directories, you can use the same kind of rule outside 
 # OASIS_START/STOP if you want to exclude directories that contains 
 # useless stuff for the build process
@@ -51,6 +51,7 @@
 "bencodeOnDisk.cmx": for-pack(Containers)
 "show.cmx": for-pack(Containers)
 "tTree.cmx": for-pack(Containers)
+"hGraph.cmx": for-pack(Containers)
 # Library containers_thread
 "containers_thread.cmxs": use_containers_thread
 <*.ml{,i}>: pkg_threads
diff --git a/containers.mlpack b/containers.mlpack
index abdffeed..3132bdde 100644
--- a/containers.mlpack
+++ b/containers.mlpack
@@ -1,5 +1,5 @@
 # OASIS_START
-# DO NOT EDIT (digest: 5c1e80992f9515200585f739a64d9771)
+# DO NOT EDIT (digest: c05f285f4d4b4832b21ddd28863f4edf)
 Cache
 Deque
 Gen
@@ -36,4 +36,5 @@ QCheck
 BencodeOnDisk
 Show
 TTree
+HGraph
 # OASIS_STOP
diff --git a/containers.odocl b/containers.odocl
index 4fd566d3..48ce7541 100644
--- a/containers.odocl
+++ b/containers.odocl
@@ -1,5 +1,5 @@
 # OASIS_START
-# DO NOT EDIT (digest: ca2e7fd09ab7b0d8b4525454fa277f3c)
+# DO NOT EDIT (digest: b68696a2df00165022fe21a389026f4f)
 Cache
 Deque
 Gen
@@ -36,5 +36,6 @@ QCheck
 BencodeOnDisk
 Show
 TTree
+HGraph
 Behavior
 # OASIS_STOP
diff --git a/hGraph.ml b/hGraph.ml
new file mode 100644
index 00000000..864c093b
--- /dev/null
+++ b/hGraph.ml
@@ -0,0 +1,255 @@
+
+(*
+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.
+*)
+
+(** {2 Hypergraph Representation}
+
+Generalized Hypergraphs. Objects are either constants, or hyperedges that
+connect [n] other objets together (a [n]-tuple). Each hyperedge can contain
+additional data.
+*)
+
+module type S = sig
+  type const
+    (** Constants. Those are what can annotate hyperedges or make single,
+        leaf, nodes. *)
+
+  type data
+    (** Additional data carried by the hypergraph elements *)
+
+  type t
+    (** An element of the hypergraph. It can be parametrized by a ['a option]
+        additional data (use 'a = unit if you don't care). *)
+
+  val eq : t -> t -> bool
+    (** Structural equality of the two edges *)
+
+  val hash : t -> int
+    (** Hash, used for hashtables *)
+
+  val id : t -> int
+    (** Same as {!hash}, but guarantees that the int is actually unique. *)
+
+  val cmp : t -> t -> int
+    (** Arbitrary total order *)
+
+  val data : t -> data option
+    (** Data contained in this edge, if any *)
+
+  val const : t -> const
+    (** Constant that annotates this hyperedge. *)
+
+  val arity : t -> int
+    (** Number of sub-elements *)
+
+  val nth : t -> int -> t
+    (** [nth x i] accesses the [i]-th sub-node of [x].
+        @raise Invalid_argument if [i >= arity x]. *)
+
+  val sub : t -> t array
+    (** Access the sub-nodes as an array. This array {b MUST NOT} be modified
+        by the caller. *)
+
+  val make : ?data:data -> const -> t array -> t
+    (** Create a new hyperedge from a constant that annotates it, and
+        an ordered tuple of sub-edges.
+        @param data optional data to decorate the edge. *)
+
+  val make_l : ?data:data -> const -> t list -> t
+    (** From a list, same as {!make} otherwise *)
+
+  val const : ?data:data -> const -> t
+    (** Constant node *)
+
+  val update : ?data:data -> t -> t array -> t
+    (** [update e sub] creates an hyperedge equivalent to [e] in all ways,
+        but with the given sub-nodes as sub-edges. The array's ownership
+        is lost by the caller.
+
+        @param data optional data that annotates the new edge.
+    *)
+
+  val pp : Buffer.t -> t -> unit
+    (** Print itself (non-recursively) on the buffer *)
+end
+
+module type PARAM = sig
+  type const
+  type data
+
+  val eq : const -> const -> bool
+  val hash : const -> int
+  val to_string : const -> string  (* for printing *)
+end
+
+module Make(P : PARAM) = struct
+  type const = P.const
+  type data = P.data
+
+  type t = {
+    head : const;
+    data : data option;
+    sub : t array;
+    mutable id : int;
+    mutable backref : t Weak.t;
+  }
+
+  type edge = t
+
+  let eq t1 t2 = t1.id = t2.id
+
+  let hash t = t.id
+
+  let id t = t.id
+
+  let cmp t1 t2 = t1.id - t2.id
+
+  let data t = t.data
+
+  let const t = t.head
+
+  let arity t = Array.length t.sub
+
+  let nth t i = t.sub.(i)
+
+  let sub t = t.sub
+
+  (* add a backref from [a] to [b]. *)
+  let _add_backref a b =
+    let n = Weak.length a.backref in
+    let arr = a.backref in
+    try
+      for i = 0 to n-1 do
+        if not (Weak.check arr i)
+          then begin  (* insert here *)
+            Weak.set arr i (Some b);
+            raise Exit;
+          end
+      done;
+      (* no insertion possible: resize *)
+      a.backref <- Weak.create (2 * n);
+      Weak.blit arr 0 a.backref 0 n;
+      Weak.set a.backref n (Some b)
+    with Exit -> ()
+
+  (* structural equality on top-level *)
+  let _eq_top t1 t2 =
+    Array.length t1.sub = Array.length t2.sub &&
+    P.eq t1.head t2.head &&
+    try
+      for i = 0 to Array.length t1.sub - 1 do
+        if not (eq (Array.unsafe_get t1.sub i) (Array.unsafe_get t2.sub i)) then raise Exit;
+      done; true
+    with Exit -> false
+
+  (* top-level hashing *)
+  let _hash_top t =
+    let h = ref (P.hash t.head) in
+    for i = 0 to Array.length t.sub - 1 do
+      h := max_int land (!h * 65599 + (hash (Array.unsafe_get t.sub i)))
+    done;
+    !h
+
+  (* hashconsing weak table *)
+  module H = Weak.Make(struct
+    type t = edge
+    let equal = _eq_top
+    let hash = _hash_top
+  end)
+
+  let __count = ref 0
+  let __table = H.create 2045
+
+  let make ?data head sub =
+    let my_t = {
+      head;
+      data;
+      sub;
+      id = ~-1;
+      backref = Weak.create 0;
+    } in
+    let t = H.merge __table my_t in
+    if t == my_t then begin
+      (* hashconsing tag *)
+      assert (t.id = ~-1);
+      t.id <- !__count;
+      incr __count;
+      (* make a proper backref array *)
+      t.backref <- Weak.create 5;
+      (* add oneself to sub-nodes' backref arrays *)
+      Array.iter (fun t' -> _add_backref t' t) sub
+      end;
+    t
+
+  let make_l ?data head sub = make ?data head (Array.of_list sub)
+
+  let const ?data head = make ?data head [| |]
+
+  let update ?data t sub' = make ?data t.head sub'
+
+  let pp buf e =
+    Buffer.add_string buf (string_of_int e.id);
+    Buffer.add_char buf ':';
+    if arity e = 0
+      then Buffer.add_string buf (P.to_string e.head)
+      else begin
+        Buffer.add_char buf '(';
+        Buffer.add_string buf (P.to_string e.head);
+        Array.iteri
+          (fun i sub ->
+            if i > 0 then Buffer.add_char buf ' ';
+            Buffer.add_string buf (string_of_int sub.id))
+          e.sub;
+        Buffer.add_char buf ')'
+      end
+end
+
+(** {2 Useful default} *)
+
+module DefaultParam = struct
+  type const =
+    | S of string
+    | I of int
+
+  type data = unit
+
+  let eq c1 c2 = match c1, c2 with
+    | S s1, S s2 -> s1 = s2
+    | I i1, I i2 -> i1 = i2
+    | _ -> false
+
+  let hash = function
+    | S s -> Hashtbl.hash s
+    | I i -> i
+
+  let to_string = function
+    | S s -> s
+    | I i -> string_of_int i
+
+  let i i = I i
+  let s s = S s
+end
+
+module Default = Make(DefaultParam)
diff --git a/hGraph.mli b/hGraph.mli
new file mode 100644
index 00000000..df4a9f7d
--- /dev/null
+++ b/hGraph.mli
@@ -0,0 +1,127 @@
+
+(*
+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.
+*)
+
+(** {2 Hypergraph Representation}
+
+Generalized Hypergraphs. Objects are either constants, or hyperedges that
+connect [n] other objets together (a [n]-tuple). Each hyperedge can contain
+additional data.
+
+Hashconsing is used to ensure that structural equality implies physical
+equality. This makes this module non thread safe.
+*)
+
+module type S = sig
+  type const
+    (** Constants. Those are what can annotate hyperedges or make single,
+        leaf, nodes. *)
+
+  type data
+    (** Additional data carried by the hypergraph elements *)
+
+  type t
+    (** An element of the hypergraph. It can be parametrized by a ['a option]
+        additional data (use 'a = unit if you don't care). *)
+
+  val eq : t -> t -> bool
+    (** Structural equality of the two edges *)
+
+  val hash : t -> int
+    (** Hash, used for hashtables *)
+
+  val id : t -> int
+    (** Same as {!hash}, but guarantees that the int is actually unique. *)
+
+  val cmp : t -> t -> int
+    (** Arbitrary total order *)
+
+  val data : t -> data option
+    (** Data contained in this edge, if any *)
+
+  val const : t -> const
+    (** Constant that annotates this hyperedge. *)
+
+  val arity : t -> int
+    (** Number of sub-elements *)
+
+  val nth : t -> int -> t
+    (** [nth x i] accesses the [i]-th sub-node of [x].
+        @raise Invalid_argument if [i >= arity x]. *)
+
+  val sub : t -> t array
+    (** Access the sub-nodes as an array. This array {b MUST NOT} be modified
+        by the caller. *)
+
+  val make : ?data:data -> const -> t array -> t
+    (** Create a new hyperedge from a constant that annotates it, and
+        an ordered tuple of sub-edges.
+        @param data optional data to decorate the edge. *)
+
+  val make_l : ?data:data -> const -> t list -> t
+    (** From a list, same as {!make} otherwise *)
+
+  val const : ?data:data -> const -> t
+    (** Constant node *)
+
+  val update : ?data:data -> t -> t array -> t
+    (** [update e sub] creates an hyperedge equivalent to [e] in all ways,
+        but with the given sub-nodes as sub-edges. The array's ownership
+        is lost by the caller.
+
+        @param data optional data that annotates the new edge.
+    *)
+
+  val pp : Buffer.t -> t -> unit
+    (** Print itself (non-recursively) on the buffer *)
+end
+
+module type PARAM = sig
+  type const
+  type data
+
+  val eq : const -> const -> bool
+  val hash : const -> int
+  val to_string : const -> string  (* for printing *)
+end
+
+module Make(P : PARAM) : S with type const = P.const and type data = P.data
+
+(** {2 Useful default} *)
+
+module DefaultParam : sig
+  type const =
+    | S of string
+    | I of int
+
+  type data = unit
+
+  include PARAM with type const := const and type data := data
+
+  val i : int -> const
+  val s : string -> const
+end
+
+module Default : S with type const = DefaultParam.const and type data = DefaultParam.data
diff --git a/setup.ml b/setup.ml
index 5a4551f2..be2a5d7e 100644
--- a/setup.ml
+++ b/setup.ml
@@ -1,7 +1,7 @@
 (* setup.ml generated for the first time by OASIS v0.3.0 *)
 
 (* OASIS_START *)
-(* DO NOT EDIT (digest: 1ff8886120b2512ffd368de480850fcc) *)
+(* DO NOT EDIT (digest: b3286e91d3acc2931801b7c5da3859c2) *)
 (*
    Regenerated by OASIS v0.3.0
    Visit http://oasis.forge.ocamlcore.org for more information and
@@ -5903,7 +5903,8 @@ let setup_t =
                            "QCheck";
                            "BencodeOnDisk";
                            "Show";
-                           "TTree"
+                           "TTree";
+                           "HGraph"
                         ];
                       lib_pack = true;
                       lib_internal_modules = [];
@@ -6224,7 +6225,7 @@ let setup_t =
           };
      oasis_fn = Some "_oasis";
      oasis_version = "0.3.0";
-     oasis_digest = Some "!l\025\004\144\144\255le\222F\177Z\202S\216";
+     oasis_digest = Some "\139{\144\225j\1545\001\221\138R\129\177\182q*";
      oasis_exec = None;
      oasis_setup_args = [];
      setup_update = false;
@@ -6232,6 +6233,6 @@ let setup_t =
 
 let setup () = BaseSetup.setup setup_t;;
 
-# 6236 "setup.ml"
+# 6237 "setup.ml"
 (* OASIS_STOP *)
 let () = setup ();;