Add map_by_2.

sequence.ml

@@ -70,6 +70,16 @@ let mapi f seq k =
   let i = ref 0 in
   seq (fun x -> k (f !i x); incr i)
 
+let map_by_2 f seq k =
+  let r = ref None in
+  let f y = match !r with
+    | None -> r := Some y
+    | Some x -> k (f x y)
+  in
+  seq f ;
+  match !r with
+  | None -> () | Some x -> k x
+
 let filter p seq k = seq (fun x -> if p x then k x)
 
 let append s1 s2 k = s1 k; s2 k

sequence.mli

@@ -109,6 +109,11 @@ val map : ('a -> 'b) -> 'a t -> 'b t
 val mapi : (int -> 'a -> 'b) -> 'a t -> 'b t
 (** Map objects, along with their index in the sequence *)
 
+val map_by_2 : ('a -> 'a -> 'a) -> 'a t -> 'a t
+  (** Map objects two by two. lazily.
+      The last element is kept in the sequence if the count is odd.
+      @since NEXT_RELEASE *)
+
 val for_all : ('a -> bool) -> 'a t -> bool
 (** Do all elements satisfy the predicate? *)
 

sequenceLabels.mli

@@ -87,6 +87,11 @@ val map : f:('a -> 'b) -> 'a t -> 'b t
 val mapi : f:(int -> 'a -> 'b) -> 'a t -> 'b t
 (** Map objects, along with their index in the sequence *)
 
+val map_by_2 : f:('a -> 'a -> 'a) -> 'a t -> 'a t
+  (** Map objects two by two. lazily.
+      The last element is kept in the sequence if the count is odd.
+      @since NEXT_RELEASE *)
+
 val for_all : f:('a -> bool) -> 'a t -> bool
 (** Do all elements satisfy the predicate? *)