val filter_map: ('a -> 'b option) -> 'a list -> 'b list (** filter + map *)
val list_concat: ?sep:'a list -> 'a list list -> 'a list (**String.concat-like*)
val filter_map: ('a -> 'b option) -> 'a list -> 'b list (** filter + map *)
val list_concat: ?sep:'a list -> 'a list list -> 'a list (**String.concat-like*)