diff --git a/containers.mllib b/containers.mllib
index 4a809eca..e9da0dd0 100644
--- a/containers.mllib
+++ b/containers.mllib
@@ -27,3 +27,4 @@ SmallSet
 Leftistheap
 AbsSet
 CSM
+MultiMap
diff --git a/multiMap.ml b/multiMap.ml
new file mode 100644
index 00000000..48e85fb0
--- /dev/null
+++ b/multiMap.ml
@@ -0,0 +1,185 @@
+
+(*
+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.
+*)
+
+(** {6 Multimap} *)
+
+module type S = sig
+  type key
+  type value
+  type t
+
+  val empty : t
+    (** Empty multimap *)
+
+  val is_empty : t -> bool
+    (** Empty multimap? *)
+
+  val add : t -> key -> value -> t
+    (** Add a key/value binding *)
+
+  val remove : t -> key -> value -> t
+    (** Remove the binding *)
+
+  val remove_all : t -> key -> t
+    (** Remove the key from the map *)
+
+  val mem : t -> key -> bool
+    (** Is there a binding for this key? *)
+
+  val find : t -> key -> value list
+    (** List of values for this key *)
+
+  val find_iter : t -> key -> (value -> unit) -> unit
+    (** Iterate on bindings for this key *)
+
+  val count : t -> key -> int
+    (** Number of bindings for this key *)
+
+  val iter : t -> (key -> value -> unit) -> unit
+    (** Iterate on all key/value *)
+
+  val fold : t -> 'a -> ('a -> key -> value -> 'a) -> 'a
+    (** Fold on all key/value *)
+
+  val size : t -> int
+    (** Number of keys *)
+
+  val union : t -> t -> t
+    (** Union of multimaps *)
+
+  val inter : t -> t -> t
+    (** Intersection of multimaps *)
+
+  val to_seq : t -> (key * value) Sequence.t
+
+  val of_seq : ?init:t -> (key * value) Sequence.t -> t
+
+  val keys : t -> key Sequence.t
+
+  val values : t -> value Sequence.t
+    (** Some values may occur several times *)
+end
+
+module type OrderedType = sig
+  type t
+  val compare : t -> t -> int
+end
+
+module Make(K : OrderedType)(V : OrderedType) = struct
+  type key = K.t
+  type value = V.t
+
+  module M = Map.Make(K)
+  module S = Set.Make(V)
+
+  type t = S.t M.t
+    (** Map of sets *)
+
+  let empty = M.empty
+
+  let is_empty = M.is_empty
+
+  let add m k v =
+    let set = try M.find k m with Not_found -> S.empty in
+    M.add k (S.add v set) m
+
+  let remove m k v =
+    try
+      let set = M.find k m in
+      let set' = S.remove v set in
+      if S.is_empty set'
+        then M.remove k m
+        else M.add k set' m
+    with Not_found ->
+      m
+
+  let remove_all m k =
+    M.remove k m
+
+  let mem m k = M.mem k m
+
+  let find m k =
+    try
+      let set = M.find k m in
+      S.elements set
+    with Not_found ->
+      []
+
+  let find_iter m k f =
+    try
+      let set = M.find k m in
+      S.iter f set
+    with Not_found ->
+      ()
+
+  let count m k =
+    try
+      let set = M.find k m in
+      S.cardinal set
+    with Not_found ->
+      0
+
+  let iter m f =
+    M.iter (fun k set -> S.iter (fun v -> f k v) set) m
+
+  let fold m acc f =
+    M.fold (fun k set acc -> S.fold (fun v acc -> f acc k v) set acc) m acc
+
+  let size m = M.cardinal m
+
+  let union m1 m2 =
+    M.merge
+      (fun k v1 v2 -> match v1, v2 with
+        | None, None -> None
+        | Some set1, Some set2 -> Some (S.union set1 set2)
+        | Some set, None
+        | None, Some set -> Some set)
+      m1 m2
+
+  let inter m1 m2 =
+    M.merge
+      (fun k v1 v2 -> match v1, v2 with
+        | None, _
+        | _, None -> None
+        | Some set1, Some set2 ->
+          let set = S.inter set1 set2 in
+          if S.is_empty set
+            then None
+            else Some set)
+      m1 m2
+
+  let to_seq m =
+    Sequence.from_iter (fun k -> iter m (fun x y -> k (x,y)))
+
+  let of_seq ?(init=empty) seq =
+    Sequence.fold (fun m (k,v) -> add m k v) init seq
+
+  let keys m =
+    Sequence.from_iter (fun k -> M.iter (fun x _ -> k x) m)
+
+  let values m =
+    Sequence.from_iter (fun k -> iter m (fun _ v -> k v))
+end
diff --git a/multiMap.mli b/multiMap.mli
new file mode 100644
index 00000000..6e498926
--- /dev/null
+++ b/multiMap.mli
@@ -0,0 +1,91 @@
+
+(*
+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.
+*)
+
+(** {6 Multimap} *)
+
+module type S = sig
+  type key
+  type value
+  type t
+
+  val empty : t
+    (** Empty multimap *)
+
+  val is_empty : t -> bool
+    (** Empty multimap? *)
+
+  val add : t -> key -> value -> t
+    (** Add a key/value binding *)
+
+  val remove : t -> key -> value -> t
+    (** Remove the binding *)
+
+  val remove_all : t -> key -> t
+    (** Remove the key from the map *)
+
+  val mem : t -> key -> bool
+    (** Is there a binding for this key? *)
+
+  val find : t -> key -> value list
+    (** List of values for this key *)
+
+  val find_iter : t -> key -> (value -> unit) -> unit
+    (** Iterate on bindings for this key *)
+
+  val count : t -> key -> int
+    (** Number of bindings for this key *)
+
+  val iter : t -> (key -> value -> unit) -> unit
+    (** Iterate on all key/value *)
+
+  val fold : t -> 'a -> ('a -> key -> value -> 'a) -> 'a
+    (** Fold on all key/value *)
+
+  val size : t -> int
+    (** Number of keys *)
+
+  val union : t -> t -> t
+    (** Union of multimaps *)
+
+  val inter : t -> t -> t
+    (** Intersection of multimaps *)
+
+  val to_seq : t -> (key * value) Sequence.t
+
+  val of_seq : ?init:t -> (key * value) Sequence.t -> t
+
+  val keys : t -> key Sequence.t
+
+  val values : t -> value Sequence.t
+    (** Some values may occur several times *)
+end
+
+module type OrderedType = sig
+  type t
+  val compare : t -> t -> int
+end
+
+module Make(K : OrderedType)(V : OrderedType) : S with type key = K.t and type value = V.t