2 files changed, 12 insertions, 0 deletions
diff --git a/lib/util.ml b/lib/util.ml
index 33f91b04ee..8b0b1e2424 100644
--- a/lib/util.ml
+++ b/lib/util.ml
@@ -214,6 +214,16 @@ let list_index x =
   in 
   index_x 1
 
+let list_unique_index x = 
+  let rec index_x n = function
+    | y::l -> 
+	if x = y then 
+	  if List.mem x l then raise Not_found
+	  else n 
+	else index_x (succ n) l
+    | [] -> raise Not_found 
+  in index_x 1
+
 let list_fold_left_i f = 
   let rec it_list_f i a = function
     | [] -> a 
diff --git a/lib/util.mli b/lib/util.mli
index f330ef8e00..1a2fedbdfc 100644
--- a/lib/util.mli
+++ b/lib/util.mli
@@ -100,6 +100,8 @@ val list_map2_i :
 val list_map3 :
   ('a -> 'b -> 'c -> 'd) -> 'a list -> 'b list -> 'c list -> 'd list
 val list_index : 'a -> 'a list -> int
+(* [list_unique_index x l] returns [Not_found] if [x] doesn't occur exactly once *)
+val list_unique_index : 'a -> 'a list -> int 
 val list_iter_i :  (int -> 'a -> unit) -> 'a list -> unit
 val list_fold_left_i :  (int -> 'a -> 'b -> 'a) -> int -> 'a -> 'b list -> 'a
 val list_fold_right_and_left :