diff --git a/README.md b/README.md
index 9fc6098e..bcc5ba01 100644
--- a/README.md
+++ b/README.md
@@ -147,6 +147,7 @@ is not necessarily up-to-date.
 - `AbsSet`, an abstract Set data structure, a bit like `LazyGraph`.
 - `Automaton`, `CSM`, state machine abstractions
 - `Bij`, a GADT-based bijection language used to serialize/deserialize your data structures
+- `Hashset`, a polymorphic imperative set on top of `PHashtbl`
 - `LazyGraph`, a lazy graph structure on arbitrary (hashable+eq) types, with basic graph functions that work even on infinite graphs, and printing to DOT.
 - `PHashtbl`, a polymorphic hashtable (with open addressing)
 - `RAL`, a random-access list structure, with `O(1)` cons/hd/tl and `O(ln(n))` access to elements by their index.
diff --git a/_oasis b/_oasis
index 9ac01b9f..e9049e20 100644
--- a/_oasis
+++ b/_oasis
@@ -113,7 +113,7 @@ Library "containers_pervasives"
 Library "containers_misc"
   Path:             src/misc
   Pack:             true
-  Modules:          AbsSet, Automaton, Bij, CSM, LazyGraph, PHashtbl,
+  Modules:          AbsSet, Automaton, Bij, CSM, Hashset, LazyGraph, PHashtbl,
                     PrintBox, RAL, RoseTree, SmallSet, UnionFind, Univ
   BuildDepends:     containers, containers.data
   FindlibName:      misc
diff --git a/_tags b/_tags
index 417c4bff..a9392af5 100644
--- a/_tags
+++ b/_tags
@@ -1,5 +1,5 @@
 # OASIS_START
-# DO NOT EDIT (digest: 5d9eb57cbb89da8bde9292277cec7a96)
+# DO NOT EDIT (digest: 2b3148849efeea95c2f260b7cce3b77e)
 # 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
@@ -55,6 +55,7 @@ true: annot, bin_annot
 "src/misc/automaton.cmx": for-pack(Containers_misc)
 "src/misc/bij.cmx": for-pack(Containers_misc)
 "src/misc/CSM.cmx": for-pack(Containers_misc)
+"src/misc/hashset.cmx": for-pack(Containers_misc)
 "src/misc/lazyGraph.cmx": for-pack(Containers_misc)
 "src/misc/pHashtbl.cmx": for-pack(Containers_misc)
 "src/misc/printBox.cmx": for-pack(Containers_misc)
diff --git a/containers.odocl b/containers.odocl
index 49a6fb00..80a5be53 100644
--- a/containers.odocl
+++ b/containers.odocl
@@ -1,5 +1,5 @@
 # OASIS_START
-# DO NOT EDIT (digest: b0ee2a2a21ef35240553e6d971c8e0b3)
+# DO NOT EDIT (digest: 2892d9c53521db701e90ccd56fbdc99a)
 src/core/CCVector
 src/core/CCPrint
 src/core/CCError
@@ -23,6 +23,7 @@ src/misc/AbsSet
 src/misc/Automaton
 src/misc/Bij
 src/misc/CSM
+src/misc/Hashset
 src/misc/LazyGraph
 src/misc/PHashtbl
 src/misc/PrintBox
diff --git a/doc/intro.txt b/doc/intro.txt
index ff7c5e99..edbad184 100644
--- a/doc/intro.txt
+++ b/doc/intro.txt
@@ -118,6 +118,7 @@ AbsSet
 Automaton
 Bij
 CSM
+Hashset
 LazyGraph
 PHashtbl
 PrintBox
diff --git a/setup.ml b/setup.ml
index 1a5912fd..25d365bf 100644
--- a/setup.ml
+++ b/setup.ml
@@ -1,7 +1,7 @@
 (* setup.ml generated for the first time by OASIS v0.4.4 *)
 
 (* OASIS_START *)
-(* DO NOT EDIT (digest: 83967354b3e0f92a4064bb798b8454ab) *)
+(* DO NOT EDIT (digest: 798266c533c11324b314d4c4bacd9a0c) *)
 (*
    Regenerated by OASIS v0.4.5
    Visit http://oasis.forge.ocamlcore.org for more information and
@@ -7409,6 +7409,7 @@ let setup_t =
                            "Automaton";
                            "Bij";
                            "CSM";
+                           "Hashset";
                            "LazyGraph";
                            "PHashtbl";
                            "PrintBox";
@@ -7935,7 +7936,7 @@ let setup_t =
        };
      oasis_fn = Some "_oasis";
      oasis_version = "0.4.5";
-     oasis_digest = Some "{v\252ox\172\235\244E\159\020\002\195\146\141\186";
+     oasis_digest = Some "\164\188^\018\173\181\135}#\017\164\201S\134\171+";
      oasis_exec = None;
      oasis_setup_args = [];
      setup_update = false
@@ -7943,6 +7944,6 @@ let setup_t =
 
 let setup () = BaseSetup.setup setup_t;;
 
-# 7947 "setup.ml"
+# 7948 "setup.ml"
 (* OASIS_STOP *)
 let () = setup ();;
diff --git a/src/misc/containers_misc.mlpack b/src/misc/containers_misc.mlpack
index 25190567..80596ad7 100644
--- a/src/misc/containers_misc.mlpack
+++ b/src/misc/containers_misc.mlpack
@@ -1,9 +1,10 @@
 # OASIS_START
-# DO NOT EDIT (digest: eb7a9d2756639dc6f89797f82adff355)
+# DO NOT EDIT (digest: 64ac3a98881a419a2ed1f076194542f9)
 AbsSet
 Automaton
 Bij
 CSM
+Hashset
 LazyGraph
 PHashtbl
 PrintBox
diff --git a/src/misc/hashset.ml b/src/misc/hashset.ml
new file mode 100644
index 00000000..110e4994
--- /dev/null
+++ b/src/misc/hashset.ml
@@ -0,0 +1,75 @@
+(*
+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 Mutable polymorphic hash-set} *)
+
+type 'a sequence = ('a -> unit) -> unit
+
+type 'a t = ('a, unit) PHashtbl.t
+  (** A set is a hashtable, with trivial values *)
+
+let empty ?max_load ?eq ?hash size =
+  PHashtbl.create ?max_load ?eq ?hash size
+
+let copy set = PHashtbl.copy set
+
+let clear set = PHashtbl.clear set
+
+let cardinal set = PHashtbl.length set
+
+let mem set x = PHashtbl.mem set x
+
+let add set x = PHashtbl.add set x ()
+
+let remove set x = PHashtbl.remove set x
+
+let iter f set = PHashtbl.iter (fun x () -> f x) set
+
+let fold f acc set = PHashtbl.fold (fun acc x () -> f acc x) acc set
+
+let filter p set = PHashtbl.filter (fun x () -> p x) set 
+
+let to_seq set k = iter k set
+
+let of_seq set seq =
+  seq (fun x -> add set x)
+
+let union ?into (s1 : 'a t) (s2 : 'a t) =
+  let into = match into with
+  | Some s -> of_seq s (to_seq s1); s
+  | None -> copy s1 in
+  of_seq into (to_seq s2);
+  into
+
+let seq_filter p seq k =
+  seq (fun x -> if p x then k x)
+
+let inter ?into (s1 : 'a t) (s2 : 'a t) =
+  let into = match into with
+  | Some s -> s
+  | None -> empty ~eq:s1.PHashtbl.eq ~hash:s1.PHashtbl.hash (cardinal s1) in
+  (* add to [into] elements of [s1] that also belong to [s2] *)
+  of_seq into (seq_filter (fun x -> mem s2 x) (to_seq s1));
+  into
diff --git a/src/misc/hashset.mli b/src/misc/hashset.mli
new file mode 100644
index 00000000..f421c557
--- /dev/null
+++ b/src/misc/hashset.mli
@@ -0,0 +1,64 @@
+(*
+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 Mutable polymorphic hash-set} *)
+
+type 'a sequence = ('a -> unit) -> unit
+
+type 'a t = ('a, unit) PHashtbl.t
+  (** A set is a hashtable, with trivial values *)
+
+val empty : ?max_load:float -> ?eq:('a -> 'a -> bool) ->
+            ?hash:('a -> int) -> int -> 'a t
+  (** See {!PHashtbl.create} *)
+
+val copy : 'a t -> 'a t
+
+val clear : 'a t -> unit
+
+val cardinal : 'a t -> int
+
+val mem : 'a t -> 'a -> bool
+
+val add : 'a t -> 'a -> unit
+
+val remove : 'a t -> 'a -> unit
+
+val iter : ('a -> unit) -> 'a t -> unit
+
+val fold : ('b -> 'a -> 'b) -> 'b -> 'a t -> 'b
+
+val filter : ('a -> bool) -> 'a t -> unit
+  (** destructive filter (remove elements that do not satisfy the predicate) *)
+
+val to_seq : 'a t -> 'a sequence
+
+val of_seq : 'a t -> 'a sequence -> unit
+
+val union : ?into:'a t -> 'a t -> 'a t -> 'a t
+  (** Set union. The result is stored in [into] *)
+
+val inter : ?into:'a t -> 'a t -> 'a t -> 'a t
+  (** Set intersection. The result is stored in [into] *)