+val flatten_map: ('a -> 'b list) -> 'a list -> 'b list
+val list_last: 'a list -> 'a
+
+ (** split_nth n l
+ * @returns two list, the first contains at least n elements, the second the
+ * remaining one
+ * @raise Failure when List.length l < n *)
+val split_nth: int -> 'a list -> 'a list * 'a list