diff --git a/src/data/CCPersistentArray.ml b/src/data/CCPersistentArray.ml
index 4929eed4..4746df78 100644
--- a/src/data/CCPersistentArray.ml
+++ b/src/data/CCPersistentArray.ml
@@ -74,20 +74,92 @@ let iteri f t = Array.iteri f (reroot t)
 let fold_left f acc t = Array.fold_left f acc (reroot t)
 let fold_right f t acc = Array.fold_right f (reroot t) acc
 
+let append a b =
+  let n = length a in
+  init (n + length b)
+    (fun i -> if i < n then get a i else get b (i-n))
+
+let flatten a =
+  let a = reroot a in
+  let n = Array.fold_left (fun acc x -> acc + length x) 0 a in
+  let i = ref 0 in  (* index in [a] *)
+  let j = ref 0 in  (* index in [a.(!i)] *)
+  init n
+    (fun _ ->
+       while !j = length a.(!i) do
+         incr i;
+         j := 0
+       done;
+       let x = get a.(!i) !j in
+       incr j;
+       x
+    )
+
+let flat_map f a =
+  let a' = map f a in
+  flatten a'
+
+(*$T
+  of_list [ of_list [1]; of_list []; of_list [2;3;4]; of_list [5]; of_list [6;7]] \
+    |> flatten |> to_list =  [1;2;3;4;5;6;7]
+  of_list [ of_list []; of_list []; of_list []] |> flatten |> length = 0
+  of_list [] |> flatten |> length = 0
+*)
+
 let to_array t = Array.copy (reroot t)
 let of_array a = init (Array.length a) (fun i -> a.(i))
 
 let to_list t = Array.to_list (reroot t)
 let of_list l = ref (Array (Array.of_list l))
 
+let rev_in_place_ a i ~len =
+  if len=0 then ()
+  else
+    for k = 0 to (len-1)/2 do
+      let t = a.(i+k) in
+      a.(i+k) <- a.(i+len-1-k);
+      a.(i+len-1-k) <- t;
+    done
+
+let of_rev_list l =
+  let a = Array.of_list l in
+  rev_in_place_ a 0 ~len:(Array.length a);
+  ref (Array a)
+
 type 'a sequence = ('a -> unit) -> unit
+type 'a gen = unit -> 'a option
 
 let to_seq a yield = iter yield a
 
 let of_seq seq =
   let l = ref [] in
   seq (fun x -> l := x :: !l);
-  of_list (List.rev !l)
+  of_rev_list !l
+
+let rec gen_iter_ f g = match g() with
+  | None -> ()
+  | Some x -> f x ; gen_iter_ f g
+
+let of_gen g =
+  let l = ref [] in
+  gen_iter_ (fun x -> l := x :: !l) g;
+  of_rev_list !l
+
+let to_gen a =
+  let i = ref 0 in
+  let n = length a in
+  fun () ->
+    if !i = n then None
+    else (
+      let x = get a !i in
+      incr i;
+      Some x
+    )
+
+(*$Q
+  Q.(list int) (fun l -> \
+    of_list l |> to_gen |> of_gen |> to_list = l)
+  *)
 
 type 'a printer = Format.formatter -> 'a -> unit
 
diff --git a/src/data/CCPersistentArray.mli b/src/data/CCPersistentArray.mli
index 57f7fb64..e333e096 100644
--- a/src/data/CCPersistentArray.mli
+++ b/src/data/CCPersistentArray.mli
@@ -84,6 +84,18 @@ val fold_left : ('a -> 'b -> 'a) -> 'a -> 'b t -> 'a
 val fold_right : ('a -> 'b -> 'b) -> 'a t -> 'b -> 'b
 (** Fold on the elements of the array. *)
 
+val append : 'a t -> 'a t -> 'a t
+(** Append the two arrays
+    @since NEXT_RELEASE *)
+
+val flatten : 'a t t -> 'a t
+(** Concatenates all the sub-arrays
+    @since NEXT_RELEASE *)
+
+val flat_map : ('a -> 'b t) -> 'a t -> 'b t
+(** Flat map (map + concatenation)
+    @since NEXT_RELEASE *)
+
 val to_array : 'a t -> 'a array
 (** [to_array t] returns a mutable copy of [t]. *)
 
@@ -96,12 +108,27 @@ val to_list : 'a t -> 'a list
 val of_list : 'a list -> 'a t
 (** [of_list l] returns a fresh persistent array containing the elements of [l]. *)
 
+val of_rev_list : 'a list -> 'a t
+(** [of_rev_list l] is the same as [of_list (List.rev l)] but more efficient
+    @since NEXT_RELEASE *)
+
+(** {2 Conversions} *)
+
 type 'a sequence = ('a -> unit) -> unit
+type 'a gen = unit -> 'a option
 
 val to_seq : 'a t -> 'a sequence
 
 val of_seq : 'a sequence -> 'a t
 
+val of_gen : 'a gen -> 'a t
+(** @since NEXT_RELEASE *)
+
+val to_gen : 'a t -> 'a gen
+(** @since NEXT_RELEASE *)
+
+(** {2 IO} *)
+
 type 'a printer = Format.formatter -> 'a -> unit
 
 val print : 'a printer -> 'a t printer