Seq: simplify the implementation of concat, concat_map

suggestion from Ulugbek Abduallev

stdlib/seq.ml

@@ -53,21 +53,12 @@ let rec filter f seq () = match seq() with
 let rec concat seq () = match seq () with
   | Nil -> Nil
   | Cons (x, next) ->
-     append_concat x next ()
-(* [append_concat seq tail] is [append seq (concat tail)] *)
-and append_concat seq tail () = match seq () with
-  | Nil -> concat tail ()
-  | Cons (x, next) -> Cons (x, append_concat next tail)
+     append x (concat next) ()
 
 let rec flat_map f seq () = match seq () with
   | Nil -> Nil
   | Cons (x, next) ->
-    append_flat_map (f x) f next ()
-(* [append_flat_map seq f tail] is [append seq (flat_map f tail)] *)
-and append_flat_map seq f tail () = match seq () with
-  | Nil -> flat_map f tail ()
-  | Cons (x, next) ->
-    Cons (x, append_flat_map next f tail)
+    append (f x) (flat_map f next) ()
 
 let concat_map = flat_map