add Sequence.pair_with_idx

src/Sequence.ml

@@ -971,6 +971,10 @@ let of_list l k = List.iter k l
 let on_list f l =
   to_list (f (of_list l))
 
+let pair_with_idx seq k =
+  let r = ref 0 in
+  seq (fun x -> let n = !r in incr r; k (n,x))
+
 let to_opt = head
 
 let of_opt o k = match o with

src/Sequence.mli

@@ -495,6 +495,10 @@ val on_list : ('a t -> 'b t) -> 'a list -> 'b list
     @since 0.5.2
 *)
 
+val pair_with_idx : 'a t -> (int * 'a) t
+(** Similar to {!zip_i} but returns a normal sequence of tuples
+    @since NEXT_RELEASE *)
+
 val to_opt : 'a t -> 'a option
 (** Alias to {!head}
     @since 0.5.1 *)