diff --git a/src/core/CCSet.ml b/src/core/CCSet.ml
index f62a96e0..0603ee73 100644
--- a/src/core/CCSet.ml
+++ b/src/core/CCSet.ml
@@ -35,10 +35,16 @@ module type S = sig
 
   val of_seq : elt sequence -> t
 
+  val add_seq : t -> elt sequence -> t
+  (** @since NEXT_RELEASE *)
+
   val to_seq : t -> elt sequence
 
   val of_list : elt list -> t
 
+  val add_list : t -> elt list -> t
+  (** @since NEXT_RELEASE *)
+
   val to_list : t -> elt list
 
   val pp : ?start:string -> ?stop:string -> ?sep:string ->
@@ -51,14 +57,18 @@ end
 module Make(O : Map.OrderedType) = struct
   include Set.Make(O)
 
-  let of_seq s =
-    let set = ref empty  in
-    s (fun x -> set := add x !set);
+  let add_seq set seq =
+    let set = ref set in
+    seq (fun x -> set := add x !set);
     !set
 
+  let of_seq s = add_seq empty s
+
   let to_seq s yield = iter yield s
 
-  let of_list l = List.fold_left (fun set x -> add x set) empty l
+  let add_list = List.fold_left (fun set x -> add x set)
+
+  let of_list l = add_list empty l
 
   let to_list = elements
 
diff --git a/src/core/CCSet.mli b/src/core/CCSet.mli
index a9b1912a..3ea014f4 100644
--- a/src/core/CCSet.mli
+++ b/src/core/CCSet.mli
@@ -37,10 +37,16 @@ module type S = sig
 
   val of_seq : elt sequence -> t
 
+  val add_seq : t -> elt sequence -> t
+  (** @since NEXT_RELEASE *)
+
   val to_seq : t -> elt sequence
 
   val of_list : elt list -> t
 
+  val add_list : t -> elt list -> t
+  (** @since NEXT_RELEASE *)
+
   val to_list : t -> elt list
 
   val pp : ?start:string -> ?stop:string -> ?sep:string ->