public GroupingOptions<G> toggleGroup(final G group) {
            if (reducedGroups.contains(group)) {
                reducedGroups.remove(group);
            } else {
                reducedGroups.add(group);
            }
            saveReducedGroups();
            triggerChange(ChangeType.GROUP_HEADER);
            return this;
        }
Sets the  minimum number of groups on which grouping is really activated
main/src/main/java/cgeo/geocaching/ui/SimpleItemListModel.java