let rec xml_of_group g =
let items = List.map xml_of_item g.group_items in
let groups = List.map xml_of_group g.group_groups in
Element (group_tag,
[att_title, g.group_title] @
(match g.group_id with
None -> []
| Some id -> [att_id, string_of_int id]
),
(items @ groups)
)