diff --git a/src/core/CCList.ml b/src/core/CCList.ml
index 48db53c0..efc614de 100644
--- a/src/core/CCList.ml
+++ b/src/core/CCList.ml
@@ -233,6 +233,28 @@ let fold_map f acc l =
     fold_map (fun acc x -> x::acc, x) [] l = (List.rev l, l))
 *)
 
+let fold_map_i f acc l =
+  let rec aux f acc i map_acc l = match l with
+    | [] -> acc, List.rev map_acc
+    | x :: l' ->
+      let acc, y = f acc i x in
+      aux f acc (i+1) (y :: map_acc) l'
+  in
+  aux f acc 0 [] l
+
+let fold_on_map ~f ~reduce acc l =
+  let rec aux acc l = match l with
+    | [] -> acc
+    | x :: l' ->
+      let acc = reduce acc (f x) in
+      aux acc l'
+  in
+  aux acc l
+
+(*$=
+  6 (fold_on_map ~f:int_of_string ~reduce:(+) 0 ["1";"2";"3"])
+*)
+
 let scan_left f acc l =
   let rec aux f acc l_acc l = match l with
     | [] -> List.rev l_acc
@@ -290,6 +312,15 @@ let fold_filter_map f acc l =
     0 (1--10))
 *)
 
+let fold_filter_map_i f acc l =
+  let rec aux f acc i map_acc l = match l with
+    | [] -> acc, List.rev map_acc
+    | x :: l' ->
+      let acc, y = f acc i x in
+      aux f acc (i+1) (cons_maybe y map_acc) l'
+  in
+  aux f acc 0 [] l
+
 let fold_flat_map f acc l =
   let rec aux f acc map_acc l = match l with
     | [] -> acc, List.rev map_acc
@@ -305,6 +336,15 @@ let fold_flat_map f acc l =
       fold_flat_map (fun acc x->acc+x, [pf "%d" x; pf "a%d" x]) 0 [1;2;3])
 *)
 
+let fold_flat_map_i f acc l =
+  let rec aux f acc i map_acc l = match l with
+    | [] -> acc, List.rev map_acc
+    | x :: l' ->
+      let acc, y = f acc i x in
+      aux f acc (i+1) (List.rev_append y map_acc) l'
+  in
+  aux f acc 0 [] l
+
 (*$Q
   Q.(list int) (fun l -> \
     fold_flat_map (fun acc x -> x::acc, [x;x+10]) [] l = \
@@ -387,6 +427,25 @@ let flat_map f l =
   List.length (flat_map (fun x->[x]) (1--300_000)) = 300_000
 *)
 
+let flat_map_i f l =
+  let rec aux f i l kont = match l with
+    | [] -> kont []
+    | x::l' ->
+      let y = f i x in
+      let kont' tail = match y with
+        | [] -> kont tail
+        | [x] -> kont (x :: tail)
+        | [x;y] -> kont (x::y::tail)
+        | l -> kont (append l tail)
+      in
+      aux f (i+1) l' kont'
+  in
+  aux f 0 l (fun l->l)
+
+(*$=
+  [1;2;2;3;3;3] (flat_map_i (fun i x->replicate (i+1) x) [1;2;3])
+*)
+
 let flatten l = fold_right append l []
 
 (*$T
diff --git a/src/core/CCList.mli b/src/core/CCList.mli
index 64430bb0..d7ea52ad 100644
--- a/src/core/CCList.mli
+++ b/src/core/CCList.mli
@@ -73,6 +73,16 @@ val fold_map : ('acc -> 'a -> 'acc * 'b) -> 'acc -> 'a list -> 'acc * 'b list
     list to another list.
     @since 0.14 *)
 
+val fold_map_i : ('acc -> int -> 'a -> 'acc * 'b) -> 'acc -> 'a list -> 'acc * 'b list
+(** [fold_map_i f init l] is a [foldi]-like function, but it also maps the
+    list to another list.
+    @since NEXT_RELEASE *)
+
+val fold_on_map : f:('a -> 'b) -> reduce:('acc -> 'b -> 'acc) -> 'acc -> 'a list -> 'acc
+(** [fold_on_map ~f ~reduce init l] combines [map f] and [fold_left reduce init]
+    in one operation.
+    @since NEXT_RELEASE *)
+
 val scan_left : ('acc -> 'a -> 'acc) -> 'acc -> 'a list -> 'acc list
 (** [scan_left f init l] returns the list [[init; f init x0; f (f init x0) x1; ...]]
     where [x0], [x1], etc. are the elements of [l].
@@ -89,11 +99,21 @@ val fold_filter_map : ('acc -> 'a -> 'acc * 'b option) -> 'acc -> 'a list -> 'ac
     generates a list of output in a way similar to {!filter_map}.
     @since 0.17 *)
 
+val fold_filter_map_i : ('acc -> int -> 'a -> 'acc * 'b option) -> 'acc -> 'a list -> 'acc * 'b list
+(** [fold_filter_map_i f init l] is a [foldi]-like function, but also
+    generates a list of output in a way similar to {!filter_map}.
+    @since NEXT_RELEASE *)
+
 val fold_flat_map : ('acc -> 'a -> 'acc * 'b list) -> 'acc -> 'a list -> 'acc * 'b list
 (** [fold_flat_map f acc l] is a [fold_left]-like function, but it also maps the
     list to a list of lists that is then [flatten]'d.
     @since 0.14 *)
 
+val fold_flat_map_i : ('acc -> int -> 'a -> 'acc * 'b list) -> 'acc -> 'a list -> 'acc * 'b list
+(** [fold_flat_map_i f acc l] is a [fold_left]-like function, but it also maps the
+    list to a list of lists that is then [flatten]'d.
+    @since NEXT_RELEASE *)
+
 val count : ('a -> bool) -> 'a list -> int
 (** [count p l] counts how many elements of [l] satisfy predicate [p].
     @since 1.5, but only
@@ -150,6 +170,12 @@ val equal : ('a -> 'a -> bool) -> 'a t -> 'a t -> bool
 val flat_map : ('a -> 'b t) -> 'a t -> 'b t
 (** Map and flatten at the same time (safe). Evaluation order is not guaranteed. *)
 
+val flat_map_i : (int -> 'a -> 'b t) -> 'a t -> 'b t
+(** Map with index and flatten at the same time (safe).
+    Evaluation order is not guaranteed.
+    @since NEXT_RELEASE
+*)
+
 val flatten : 'a t t -> 'a t
 (** Safe flatten. Concatenate a list of lists. *)
 
diff --git a/src/core/CCListLabels.mli b/src/core/CCListLabels.mli
index bc914720..742cee5d 100644
--- a/src/core/CCListLabels.mli
+++ b/src/core/CCListLabels.mli
@@ -73,6 +73,16 @@ val fold_map : f:('acc -> 'a -> 'acc * 'b) -> init:'acc -> 'a list -> 'acc * 'b
     list to another list.
     @since 0.14 *)
 
+val fold_map_i : f:('acc -> int -> 'a -> 'acc * 'b) -> init:'acc -> 'a list -> 'acc * 'b list
+(** [fold_map_i f init l] is a [foldi]-like function, but it also maps the
+    list to another list.
+    @since NEXT_RELEASE *)
+
+val fold_on_map : f:('a -> 'b) -> reduce:('acc -> 'b -> 'acc) -> init:'acc -> 'a list -> 'acc
+(** [fold_on_map ~f ~reduce init l] combines [map f] and [fold_left reduce init]
+    in one operation.
+    @since NEXT_RELEASE *)
+
 val scan_left : f:('acc -> 'a -> 'acc) -> init:'acc -> 'a list -> 'acc list
 (** [scan_left ~f ~init l] returns the list [[init; f init x0; f (f init x0) x1; ...]]
     where [x0], [x1], etc. are the elements of [l].
@@ -89,11 +99,21 @@ val fold_filter_map : f:('acc -> 'a -> 'acc * 'b option) -> init:'acc -> 'a list
     generates a list of output in a way similar to {!filter_map}.
     @since 0.17 *)
 
+val fold_filter_map_i : f:('acc -> int -> 'a -> 'acc * 'b option) -> init:'acc -> 'a list -> 'acc * 'b list
+(** [fold_filter_map_i f init l] is a [foldi]-like function, but also
+    generates a list of output in a way similar to {!filter_map}.
+    @since NEXT_RELEASE *)
+
 val fold_flat_map : f:('acc -> 'a -> 'acc * 'b list) -> init:'acc -> 'a list -> 'acc * 'b list
 (** [fold_flat_map f acc l] is a [fold_left]-like function, but it also maps the
     list to a list of lists that is then [flatten]'d.
     @since 0.14 *)
 
+val fold_flat_map_i : f:('acc -> int -> 'a -> 'acc * 'b list) -> init:'acc -> 'a list -> 'acc * 'b list
+(** [fold_flat_map_i f acc l] is a [fold_left]-like function, but it also maps the
+    list to a list of lists that is then [flatten]'d.
+    @since NEXT_RELEASE *)
+
 val count : f:('a -> bool) -> 'a list -> int
 (** [count p l] counts how many elements of [l] satisfy predicate [p].
     @since 1.5, but only
@@ -150,6 +170,12 @@ val equal : ('a -> 'a -> bool) -> 'a t -> 'a t -> bool
 val flat_map : f:('a -> 'b t) -> 'a t -> 'b t
 (** Map and flatten at the same time (safe). Evaluation order is not guaranteed. *)
 
+val flat_map_i : f:(int -> 'a -> 'b t) -> 'a t -> 'b t
+(** Map with index and flatten at the same time (safe).
+    Evaluation order is not guaranteed.
+    @since NEXT_RELEASE
+*)
+
 val flatten : 'a t t -> 'a t
 (** Safe flatten. Concatenate a list of lists. *)
 
diff --git a/src/data/CCGraph.mli b/src/data/CCGraph.mli
index f711003f..abf43210 100644
--- a/src/data/CCGraph.mli
+++ b/src/data/CCGraph.mli
@@ -40,7 +40,7 @@ type 'a sequence = ('a -> unit) -> unit
     @deprecate see {!iter} instead *)
 [@@ocaml.deprecated "see iter"]
 
-type 'a sequence_once = 'a sequence
+type 'a sequence_once = 'a iter
 (** Iter that should be used only once
     @deprecate see {!iter_once} instead *)
 [@@ocaml.deprecated "see iter_once"]