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
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