public GroupingOptions<G> setReducedGroupSaver(final String saveId, final Function<G, String> saveGroupMapper, final Function<String, G> saveGroupBackMapper) {
            this.reducedGroupSaveId = saveId;
            this.reducedGroupIdMapper = saveGroupMapper;
            this.reducedGroupIdBackMapper = saveGroupBackMapper;
            loadReducedGroups();
            return this;
        }
Sets the  minimum number of groups on which grouping is really activated
main/src/main/java/cgeo/geocaching/ui/SimpleItemListModel.java