2 files changed, 10 insertions, 0 deletions
diff --git a/lib/cList.ml b/lib/cList.ml
index bd3e09b5b2..ba592d13f3 100644
--- a/lib/cList.ml
+++ b/lib/cList.ml
@@ -48,6 +48,7 @@ sig
   val filteri :
     (int -> 'a -> bool) -> 'a list -> 'a list
   val smartfilter : ('a -> bool) -> 'a list -> 'a list
+  val extend : bool list -> 'a -> 'a list -> 'a list
   val count : ('a -> bool) -> 'a list -> int
   val index : 'a eq -> 'a -> 'a list -> int
   val index0 : 'a eq -> 'a -> 'a list -> int
@@ -376,6 +377,12 @@ let rec smartfilter f l = match l with
           else h :: tl'
         else tl'
 
+let rec extend l a l' = match l,l' with
+  | true::l, b::l' -> b :: extend l a l'
+  | false::l, l' -> a :: extend l a l'
+  | [], [] -> []
+  | _ -> invalid_arg "extend"
+
 let count f l =
   let rec aux acc = function
     | [] -> acc
diff --git a/lib/cList.mli b/lib/cList.mli
index 1487f67a37..9c7b815c15 100644
--- a/lib/cList.mli
+++ b/lib/cList.mli
@@ -94,6 +94,9 @@ sig
   (** [smartfilter f [a1...an] = List.filter f [a1...an]] but if for all i
     [f ai = true], then [smartfilter f l == l] *)
 
+  val extend : bool list -> 'a -> 'a list -> 'a list
+(** [extend l a [a1..an]] assumes that the number of [true] in [l] is [n];
+    it extends [a1..an] by inserting [a] at the position of [false] in [l] *)
   val count : ('a -> bool) -> 'a list -> int
 
   val index : 'a eq -> 'a -> 'a list -> int