diff --git a/hashset.ml b/hashset.ml
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+++ b/hashset.ml
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+(*
+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 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 =
+  Sequence.from_iter (fun k -> iter k set)
+
+let of_seq set seq =
+  Sequence.iter (fun x -> add set x) seq
+
+let union s1 s2 =
+  of_seq s1 (to_seq s2)
+
+let inter s1 s2 =
+  let set = copy s1 in
+  filter (fun x -> mem s2 x) set;
+  set
diff --git a/hashset.mli b/hashset.mli
new file mode 100644
index 00000000..c56c5ce5
--- /dev/null
+++ b/hashset.mli
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+(*
+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 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 {@see PHashtbl.create} *)
+
+val copy : 'a t -> 'a t
+
+val clear : 'a t -> 'a t
+
+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.t
+
+val of_seq : 'a t -> 'a Sequence.t -> unit
+
+val union : 'a t -> 'a t -> 'a t
+
+val inter : 'a t -> 'a t -> 'a t