val filter_map: ('a -> 'b option) -> 'a list -> 'b list (** filter + map *)
val filter_map_acc: ('acc -> 'a -> ('acc * 'b) option) -> 'acc -> 'a list ->
'acc * 'b list (** fold/filter + map *)
val filter_map: ('a -> 'b option) -> 'a list -> 'b list (** filter + map *)
val filter_map_acc: ('acc -> 'a -> ('acc * 'b) option) -> 'acc -> 'a list ->
'acc * 'b list (** fold/filter + map *)