val failwithf : ('a, unit, string, 'b) format4 -> 'a
(** Like [failwith] but supports printf-like arguments. *)
+
+val filter_map : ('a -> 'b option) -> 'a list -> 'b list
+(** [filter_map f l] applies [f] to every element of [l], filters
+ out the [None] elements and returns the list of the arguments of
+ the [Some] elements. *)