diff --git a/src/core/CCHashtbl.ml b/src/core/CCHashtbl.ml
index 66971f1b..a2ec9922 100644
--- a/src/core/CCHashtbl.ml
+++ b/src/core/CCHashtbl.ml
@@ -30,6 +30,10 @@ let values tbl k = Hashtbl.iter (fun _ v -> k v) tbl
 let keys_list tbl = Hashtbl.fold (fun k _ a -> k::a) tbl []
 let values_list tbl = Hashtbl.fold (fun _ v a -> v::a) tbl []
 
+let add_list tbl k v =
+  let l = try Hashtbl.find tbl k with Not_found -> [] in
+  Hashtbl.replace tbl k (v::l)
+
 let incr ?(by=1) tbl x =
   let n = get_or tbl x ~or_:0 in
   if n+by <= 0
@@ -125,6 +129,11 @@ module type S = sig
       and returns [or_] otherwise (if [k] doesn't belong in [tbl])
       @since NEXT_RELEASE *)
 
+  val add_list : 'a list t -> key -> 'a -> unit
+  (** [add_list tbl x y] adds [y] to the list [x] is bound to. If [x] is
+      not bound, it becomes bound to [[y]].
+      @since NEXT_RELEASE *)
+
   val incr : ?by:int -> int t -> key -> unit
   (** [incr ?by tbl x] increments or initializes the counter associated with [x].
       If [get tbl x = None], then after update, [get tbl x = Some 1];
@@ -236,6 +245,10 @@ module Make(X : Hashtbl.HashedType)
     assert_bool "2 removed" (not (T.mem tbl 2));
   *)
 
+  let add_list tbl k v =
+    let l = try find tbl k with Not_found -> [] in
+    replace tbl k (v::l)
+
   let decr ?(by=1) tbl x =
     try
       let n = find tbl x in
diff --git a/src/core/CCHashtbl.mli b/src/core/CCHashtbl.mli
index b43e4b33..2e51e6bd 100644
--- a/src/core/CCHashtbl.mli
+++ b/src/core/CCHashtbl.mli
@@ -53,6 +53,11 @@ val decr : ?by:int -> ('a, int) Hashtbl.t -> 'a -> unit
 val to_seq : ('a,'b) Hashtbl.t -> ('a * 'b) sequence
 (** Iterate on bindings in the table *)
 
+val add_list : ('a, 'b list) Hashtbl.t -> 'a -> 'b -> unit
+(** [add_list tbl x y] adds [y] to the list [x] is bound to. If [x] is
+    not bound, it becomes bound to [[y]].
+    @since NEXT_RELEASE *)
+
 val add_seq : ('a,'b) Hashtbl.t -> ('a * 'b) sequence -> unit
 (** Add the corresponding pairs to the table, using {!Hashtbl.add}.
     @since NEXT_RELEASE *)
@@ -101,6 +106,11 @@ module type S = sig
       and returns [or_] otherwise (if [k] doesn't belong in [tbl])
       @since NEXT_RELEASE *)
 
+  val add_list : 'a list t -> key -> 'a -> unit
+  (** [add_list tbl x y] adds [y] to the list [x] is bound to. If [x] is
+      not bound, it becomes bound to [[y]].
+      @since NEXT_RELEASE *)
+
   val incr : ?by:int -> int t -> key -> unit
   (** [incr ?by tbl x] increments or initializes the counter associated with [x].
       If [get tbl x = None], then after update, [get tbl x = Some 1];