public GroupingOptions<G> setGroupGroupMapper(final Function<G, G> groupGroupMapper) {
            this.groupGroupMapper = groupGroupMapper;
            triggerChange(ChangeType.COMPLETE);
            return this;
        }
Sets a mapper specifying how group items (of type G) are sorted into other groups (of type G)
main/src/main/java/cgeo/geocaching/ui/SimpleItemListModel.java