diff --git a/src/core/CCMap.ml b/src/core/CCMap.ml
index 49b59b46..7df8dc58 100644
--- a/src/core/CCMap.ml
+++ b/src/core/CCMap.ml
@@ -12,7 +12,9 @@ module type S = sig
   include Map.S
 
   val get : key -> 'a t -> 'a option
-  (** Safe version of {!find}. *)
+  (** [get k m] returns [Some v] if the current binding of [k] in [m] is [v],
+      or [None] if the key [k] is not present.
+      Safe version of {!find}. *)
 
   val get_or : key -> 'a t -> default:'a -> 'a
   (** [get_or k m ~default] returns the value associated to [k] if present,
@@ -26,28 +28,38 @@ module type S = sig
       [add k v' m] is returned. *)
 
   val choose_opt : 'a t -> (key * 'a) option
-  (** Safe version of {!choose}.
+  (** [choose_opt m] returns one binding of the given map [m], or [None] if [m] is empty.
+      Safe version of {!choose}.
       @since 1.5 *)
 
   val min_binding_opt : 'a t -> (key * 'a) option
-  (** Safe version of {!min_binding}.
+  (** [min_binding_opt m] returns the smallest binding of the given map [m],
+      or [None] if [m] is empty.
+      Safe version of {!min_binding}.
       @since 1.5 *)
 
   val max_binding_opt : 'a t -> (key * 'a) option
-  (** Safe version of {!max_binding}.
+  (** [max_binding_opt m] returns the largest binding of the given map [m],
+      or [None] if [m] is empty.
+      Safe version of {!max_binding}.
       @since 1.5 *)
 
   val find_opt : key -> 'a t -> 'a option
-  (** Safe version of {!find}.
+  (** [find_opt k m] returns [Some v] if the current binding of [k] in [m] is [v],
+      or [None] if the key [k] is not present.
+      Safe version of {!find}.
       @since 1.5 *)
 
   val find_first : (key -> bool) -> 'a t -> key * 'a
-  (** Find smallest binding satisfying the monotonic predicate.
+  (** [find_first f m] where [f] is a monotonically increasing function, returns the binding of [m]
+      with the lowest key [k] such that [f k], or raises [Not_found] if no such key exists.
       See {!Map.S.find_first}.
       @since 1.5 *)
 
   val find_first_opt : (key -> bool) -> 'a t -> (key * 'a) option
-  (** Safe version of {!find_first}.
+  (** [find_first_opt f m] where [f] is a monotonically increasing function, returns an option containing
+      the binding of [m] with the lowest key [k] such that [f k], or [None] if no such key exists.
+      Safe version of {!find_first}.
       @since 1.5 *)
 
   val merge_safe :
@@ -56,51 +68,98 @@ module type S = sig
   (** [merge_safe ~f a b] merges the maps [a] and [b] together.
       @since 0.17 *)
 
-  val of_iter : (key * 'a) iter -> 'a t
-  (** Like {!of_list}.
-      @since 2.8 *)
-
   val add_seq : 'a t -> (key * 'a) Seq.t -> 'a t
-  (** Like {!add_list}.
-      @since 2.8 *)
+  (** [add_seq m seq] adds the given [Seq.t] of bindings to the map [m].
+      Like {!add_list}.
+      Renamed from [add_std_seq] since 3.0.
+      @since 3.0 *)
+
+  val add_seq_with : f:(key -> 'a -> 'a -> 'a) -> 'a t -> (key * 'a) Seq.t -> 'a t
+  (** [add_seq ~f m l] adds the given seq [l] of bindings to the map [m],
+      using [f] to combine values that have the same key.
+      @since NEXT_RELEASE *)
 
   val of_seq : (key * 'a) Seq.t -> 'a t
-  (** Like {!of_list}.
-      @since 2.8 *)
+  (** [of_seq seq] builds a map from the given [Seq.t] of bindings.
+      Like {!of_list}.
+      Renamed from [of_std_seq] since 3.0.
+      @since 3.0 *)
+
+  val of_seq_with : f:(key -> 'a -> 'a -> 'a) -> (key * 'a) Seq.t -> 'a t
+  (** [of_seq_with ~f l] builds a map from the given seq [l] of bindings [k_i -> v_i],
+      added in order using {!add}.
+      If a key occurs several times, all its bindings are combined using the
+      function [f], with [f key v1 v2] being called with [v1] occurring
+      later in the seq than [v2].
+      @since NEXT_RELEASE *)
 
   val add_iter : 'a t -> (key * 'a) iter -> 'a t
-  (** Like {!add_list}.
+  (** [add_iter m iter] adds the given [iter] of bindings to the map [m].
+      Like {!add_list}.
       @since 2.8 *)
 
+  val add_iter_with : f:(key -> 'a -> 'a -> 'a) -> 'a t -> (key * 'a) iter -> 'a t
+  (** [add_iter ~f m l] adds the given iter [l] of bindings to the map [m],
+      using [f] to combine values that have the same key.
+      @since NEXT_RELEASE *)
+
   val of_iter : (key * 'a) iter -> 'a t
-  (** Like {!of_list}.
+  (** [of_iter iter] builds a map from the given [iter] of bindings.
+      Like {!of_list}.
       @since 2.8 *)
 
+  val of_iter_with : f:(key -> 'a -> 'a -> 'a) -> (key * 'a) iter -> 'a t
+  (** [of_iter_with ~f l] builds a map from the given iter [l] of bindings [k_i -> v_i],
+      added in order using {!add}.
+      If a key occurs several times, all its bindings are combined using the
+      function [f], with [f key v1 v2] being called with [v1] occurring
+      later in the iter than [v2].
+      @since NEXT_RELEASE *)
+
   val to_iter : 'a t -> (key * 'a) iter
-  (** Like {!to_list}.
+  (** [to_iter m] iterates on the whole map [m], creating an [iter] of bindings.
+      Like {!to_list}.
       @since 2.8 *)
 
   val of_list : (key * 'a) list -> 'a t
-  (** Build a map from the given list of bindings [k_i -> v_i],
+  (** [of_list l] builds a map from the given list [l] of bindings [k_i -> v_i],
       added in order using {!add}.
       If a key occurs several times, only its last binding
       will be present in the result. *)
 
+  val of_list_with : f:(key -> 'a -> 'a -> 'a) -> (key * 'a) list -> 'a t
+  (** [of_list_with ~f l] builds a map from the given list [l] of bindings [k_i -> v_i],
+      added in order using {!add}.
+      If a key occurs several times, all its bindings are combined using the
+      function [f], with [f key v1 v2] being called with [v1] occurring
+      later in the list than [v2].
+      @since NEXT_RELEASE *)
+
   val add_list : 'a t -> (key * 'a) list -> 'a t
-  (** @since 0.14 *)
+  (** [add_list m l] adds the given list [l] of bindings to the map [m].
+      @since 0.14 *)
+
+  val add_list_with : f:(key -> 'a -> 'a -> 'a) -> 'a t -> (key * 'a) list -> 'a t
+  (** [add_list ~f m l] adds the given list [l] of bindings to the map [m],
+      using [f] to combine values that have the same key.
+      @since NEXT_RELEASE *)
 
   val keys : _ t -> key iter
-  (** Iterate on keys only.
+  (** [keys m] iterates on the keys of [m] only, creating an [iter] of keys.
       @since 0.15 *)
 
   val values : 'a t -> 'a iter
-  (** Iterate on values only.
+  (** [values m] iterates on the values of [m] only, creating an [iter] of values.
       @since 0.15 *)
 
   val to_list : 'a t -> (key * 'a) list
+  (** [to_list m] builds a list of the bindings of the given map [m].
+      The order is unspecified. *)
 
   val pp : ?pp_start:unit printer -> ?pp_stop:unit printer -> ?pp_arrow:unit printer ->
     ?pp_sep:unit printer -> key printer -> 'a printer -> 'a t printer
+  (** [pp ?pp_start ?pp_stop ?pp_arrow ?pp_sep pp_key pp_v m] pretty-prints the
+      contents of the map. *)
 end
 
 (*$inject
@@ -210,14 +269,30 @@ module Make(O : Map.OrderedType) = struct
     Seq.iter (fun (k,v) -> m := add k v !m) s;
     !m
 
+  let add_seq_with ~f m s =
+    let m = ref m in
+    Seq.iter (fun (k,v) ->
+        let v = try f k v (find k !m) with Not_found -> v in
+        m := add k v !m) s;
+    !m
+
   let of_seq s = add_seq empty s
+  let of_seq_with ~f s = add_seq_with ~f empty s
 
   let add_iter m s =
     let m = ref m in
     s (fun (k,v) -> m := add k v !m);
     !m
 
+  let add_iter_with ~f m s =
+    let m = ref m in
+    s (fun (k,v) ->
+        let v = try f k v (find k !m) with Not_found -> v in
+        m := add k v !m);
+    !m
+
   let of_iter s = add_iter empty s
+  let of_iter_with ~f s = add_iter_with ~f empty s
 
   let to_iter m yield =
     iter (fun k v -> yield (k,v)) m
@@ -229,8 +304,15 @@ module Make(O : Map.OrderedType) = struct
     iter (fun _ v -> yield v) m
 
   let add_list m l = List.fold_left (fun m (k,v) -> add k v m) m l
+  let add_list_with ~f m l =
+    List.fold_left
+      (fun m (k,v) ->
+         let v = try f k v (find k m) with Not_found -> v in
+         add k v m)
+      m l
 
   let of_list l = add_list empty l
+  let of_list_with ~f l = add_list_with ~f empty l
 
   let to_list m =
     fold (fun k v acc -> (k,v)::acc) m []
@@ -250,3 +332,18 @@ module Make(O : Map.OrderedType) = struct
       m;
     pp_stop fmt ()
 end
+
+(*$inject
+  module M2 = Make(CCInt)
+*)
+
+(*$Q
+  Q.(list (pair small_int small_int)) M2.(fun l -> \
+    to_list (of_list l) = to_list (of_list_with ~f:(fun _ v _ ->v) l))
+  Q.(list (pair small_int small_int)) M2.(fun l -> \
+   to_list (of_iter @@ Iter.of_list l) = \
+   to_list (of_iter_with ~f:(fun _ v _ ->v) @@ Iter.of_list l))
+  Q.(list (pair small_int small_int)) M2.(fun l -> \
+   to_list (of_seq @@ CCSeq.of_list l) = \
+   to_list (of_seq_with ~f:(fun _ v _ ->v) @@ CCSeq.of_list l))
+*)
diff --git a/src/core/CCMap.mli b/src/core/CCMap.mli
index 4797e449..a7ebacb6 100644
--- a/src/core/CCMap.mli
+++ b/src/core/CCMap.mli
@@ -75,33 +75,54 @@ module type S = sig
   (** [merge_safe ~f a b] merges the maps [a] and [b] together.
       @since 0.17 *)
 
-  val of_iter : (key * 'a) iter -> 'a t
-  (** [of_iter iter] builds a map from the given [iter] of bindings.
-      Like {!of_list}.
-      @since 2.8 *)
-
   val add_seq : 'a t -> (key * 'a) Seq.t -> 'a t
   (** [add_seq m seq] adds the given [Seq.t] of bindings to the map [m].
       Like {!add_list}.
       Renamed from [add_std_seq] since 3.0.
       @since 3.0 *)
 
+  val add_seq_with : f:(key -> 'a -> 'a -> 'a) -> 'a t -> (key * 'a) Seq.t -> 'a t
+  (** [add_seq ~f m l] adds the given seq [l] of bindings to the map [m],
+      using [f] to combine values that have the same key.
+      @since NEXT_RELEASE *)
+
   val of_seq : (key * 'a) Seq.t -> 'a t
   (** [of_seq seq] builds a map from the given [Seq.t] of bindings.
       Like {!of_list}.
       Renamed from [of_std_seq] since 3.0.
       @since 3.0 *)
 
+  val of_seq_with : f:(key -> 'a -> 'a -> 'a) -> (key * 'a) Seq.t -> 'a t
+  (** [of_seq_with ~f l] builds a map from the given seq [l] of bindings [k_i -> v_i],
+      added in order using {!add}.
+      If a key occurs several times, all its bindings are combined using the
+      function [f], with [f key v1 v2] being called with [v1] occurring
+      later in the seq than [v2].
+      @since NEXT_RELEASE *)
+
   val add_iter : 'a t -> (key * 'a) iter -> 'a t
   (** [add_iter m iter] adds the given [iter] of bindings to the map [m].
       Like {!add_list}.
       @since 2.8 *)
 
+  val add_iter_with : f:(key -> 'a -> 'a -> 'a) -> 'a t -> (key * 'a) iter -> 'a t
+  (** [add_iter ~f m l] adds the given iter [l] of bindings to the map [m],
+      using [f] to combine values that have the same key.
+      @since NEXT_RELEASE *)
+
   val of_iter : (key * 'a) iter -> 'a t
   (** [of_iter iter] builds a map from the given [iter] of bindings.
       Like {!of_list}.
       @since 2.8 *)
 
+  val of_iter_with : f:(key -> 'a -> 'a -> 'a) -> (key * 'a) iter -> 'a t
+  (** [of_iter_with ~f l] builds a map from the given iter [l] of bindings [k_i -> v_i],
+      added in order using {!add}.
+      If a key occurs several times, all its bindings are combined using the
+      function [f], with [f key v1 v2] being called with [v1] occurring
+      later in the iter than [v2].
+      @since NEXT_RELEASE *)
+
   val to_iter : 'a t -> (key * 'a) iter
   (** [to_iter m] iterates on the whole map [m], creating an [iter] of bindings.
       Like {!to_list}.
@@ -113,10 +134,23 @@ module type S = sig
       If a key occurs several times, only its last binding
       will be present in the result. *)
 
+  val of_list_with : f:(key -> 'a -> 'a -> 'a) -> (key * 'a) list -> 'a t
+  (** [of_list_with ~f l] builds a map from the given list [l] of bindings [k_i -> v_i],
+      added in order using {!add}.
+      If a key occurs several times, all its bindings are combined using the
+      function [f], with [f key v1 v2] being called with [v1] occurring
+      later in the list than [v2].
+      @since NEXT_RELEASE *)
+
   val add_list : 'a t -> (key * 'a) list -> 'a t
   (** [add_list m l] adds the given list [l] of bindings to the map [m].
       @since 0.14 *)
 
+  val add_list_with : f:(key -> 'a -> 'a -> 'a) -> 'a t -> (key * 'a) list -> 'a t
+  (** [add_list ~f m l] adds the given list [l] of bindings to the map [m],
+      using [f] to combine values that have the same key.
+      @since NEXT_RELEASE *)
+
   val keys : _ t -> key iter
   (** [keys m] iterates on the keys of [m] only, creating an [iter] of keys.
       @since 0.15 *)