val gwb2ged :
  bool ->
  Gwexport.gwexport_opts ->
  (Gwdb.iper -> bool) * (Gwdb.ifam -> bool) ->
  unit
(** [gwb2ged with_indexes opts sel]
    Converts a Geneweb database to a GEDCOM file.
    * `with_indexes` specifies if indexes are printed or not;
    * `opts` are the export options
    * `sel` is a pair of selectors returned by the database export
*)