diff --git a/src/Sequence.ml b/src/Sequence.ml
index 485a7b2..da7daa8 100644
--- a/src/Sequence.ml
+++ b/src/Sequence.ml
@@ -15,6 +15,9 @@ type (+'a, +'b) t2 = ('a -> 'b -> unit) -> unit
   let pp_ilist = Q.Print.(list int)
 *)
 
+type 'a equal = 'a -> 'a -> bool
+type 'a hash = 'a -> int
+
 (** Build a sequence from a iter function *)
 let from_iter f = f
 
@@ -521,6 +524,96 @@ let join ~join_row s1 s2 k =
   OUnit.assert_equal ["1 = 1"; "2 = 2"] (to_list s);
 *)
 
+let join_by (type a) ?(eq=(=)) ?(hash=Hashtbl.hash) f1 f2 ~merge c1 c2 =
+  let module Tbl = Hashtbl.Make(struct
+      type t = a
+      let equal = eq
+      let hash = hash
+    end) in
+  let tbl = Tbl.create 32 in
+  c1
+    (fun x ->
+       let key = f1 x in
+       Tbl.add tbl key x);
+  let res = ref [] in
+  c2
+    (fun y ->
+       let key = f2 y in
+       let xs = Tbl.find_all tbl key in
+       List.iter
+         (fun x -> match merge key x y with
+            | None -> ()
+            | Some z -> res := z :: !res)
+         xs);
+  fun yield -> List.iter yield !res
+
+type ('a, 'b) join_all_cell = {
+  mutable ja_left: 'a list;
+  mutable ja_right: 'b list;
+}
+
+let join_all_by (type a) ?(eq=(=)) ?(hash=Hashtbl.hash) f1 f2 ~merge c1 c2 =
+  let module Tbl = Hashtbl.Make(struct
+      type t = a
+      let equal = eq
+      let hash = hash
+    end) in
+  let tbl = Tbl.create 32 in
+  (* build the map [key -> cell] *)
+  c1
+    (fun x ->
+       let key = f1 x in
+       try
+         let c = Tbl.find tbl key in
+         c.ja_left <- x :: c.ja_left
+       with Not_found ->
+         Tbl.add tbl key {ja_left=[x]; ja_right=[]});
+  c2
+    (fun y ->
+       let key = f2 y in
+       try
+         let c = Tbl.find tbl key in
+         c.ja_right <- y :: c.ja_right
+       with Not_found ->
+         Tbl.add tbl key {ja_left=[]; ja_right=[y]});
+  let res = ref [] in
+  Tbl.iter
+    (fun key cell -> match merge key cell.ja_left cell.ja_right with
+       | None -> ()
+       | Some z -> res := z :: !res)
+    tbl;
+  fun yield -> List.iter yield !res
+
+let group_join_by (type a) ?(eq=(=)) ?(hash=Hashtbl.hash) f c1 c2 =
+  let module Tbl = Hashtbl.Make(struct
+      type t = a
+      let equal = eq
+      let hash = hash
+    end) in
+  let tbl = Tbl.create 32 in
+  c1 (fun x -> Tbl.replace tbl x []);
+  c2
+    (fun y ->
+       (* project [y] into some element of [c1] *)
+       let key = f y in
+       try
+         let l = Tbl.find tbl key in
+         Tbl.replace tbl key (y :: l)
+       with Not_found -> ());
+  fun yield -> Tbl.iter (fun k l -> yield (k,l)) tbl
+
+(*$=
+  ['a', ["abc"; "attic"]; \
+   'b', ["barbary"; "boom"; "bop"]; \
+   'c', []] \
+  (group_join_by (fun s->s.[0]) \
+    (of_str "abc") \
+    (of_list ["abc"; "boom"; "attic"; "deleted"; "barbary"; "bop"]) \
+  |> map (fun (c,l)->c,List.sort Pervasives.compare l) \
+  |> sort |> to_list)
+*)
+
+
 let rec unfoldr f b k = match f b with
   | None -> ()
   | Some (x, b') ->
diff --git a/src/Sequence.mli b/src/Sequence.mli
index 15d394a..1d6509a 100644
--- a/src/Sequence.mli
+++ b/src/Sequence.mli
@@ -39,6 +39,9 @@ type +'a sequence = 'a t
 type (+'a, +'b) t2 = ('a -> 'b -> unit) -> unit
 (** Sequence of pairs of values of type ['a] and ['b]. *)
 
+type 'a equal = 'a -> 'a -> bool
+type 'a hash = 'a -> int
+
 (** {2 Build a sequence} *)
 
 val from_iter : (('a -> unit) -> unit) -> 'a t
@@ -285,6 +288,49 @@ val join : join_row:('a -> 'b -> 'c option) -> 'a t -> 'b t -> 'c t
     the two elements do not combine. Assume that [b] allows for multiple
     iterations. *)
 
+val join_by : ?eq:'key equal -> ?hash:'key hash ->
+  ('a -> 'key) -> ('b -> 'key) ->
+  merge:('key -> 'a -> 'b -> 'c option) ->
+  'a t ->
+  'b t ->
+  'c t
+(** [join key1 key2 ~merge] is a binary operation
+    that takes two sequences [a] and [b], projects their
+    elements resp. with [key1] and [key2], and combine
+    values [(x,y)] from [(a,b)] with the same [key]
+    using [merge]. If [merge] returns [None], the combination
+    of values is discarded.
+    @since NEXT_RELEASE *)
+
+val join_all_by : ?eq:'key equal -> ?hash:'key hash ->
+  ('a -> 'key) -> ('b -> 'key) ->
+  merge:('key -> 'a list -> 'b list -> 'c option) ->
+  'a t ->
+  'b t ->
+  'c t
+(** [join_all_by key1 key2 ~merge] is a binary operation
+    that takes two sequences [a] and [b], projects their
+    elements resp. with [key1] and [key2], and, for each key [k]
+    occurring in at least one of them:
+    - compute the list [l1] of elements of [a] that map to [k]
+    - compute the list [l2] of elements of [b] that map to [k]
+    - call [merge k l1 l2]. If [merge] returns [None], the combination
+      of values is discarded, otherwise it returns [Some c]
+      and [c] is inserted in the result.
+    @since NEXT_RELEASE *)
+
+val group_join_by : ?eq:'a equal -> ?hash:'a hash ->
+  ('b -> 'a) ->
+  'a t ->
+  'b t ->
+  ('a * 'b list) t
+(** [group_join_by key2] associates to every element [x] of
+    the first sequence, all the elements [y] of the second
+    sequence such that [eq x (key y)]. Elements of the first
+    sequences without corresponding values in the second one
+    are mapped to [[]]
+    @since NEXT_RELEASE *)
+
 val unfoldr : ('b -> ('a * 'b) option) -> 'b -> 'a t
 (** [unfoldr f b] will apply [f] to [b]. If it
     yields [Some (x,b')] then [x] is returned