public GroupingOptions<G> setReducedGroups(final Iterable<G> reducedGroups) {
            this.reducedGroups.clear();
            if (reducedGroups != null) {
                for (G group : reducedGroups) {
                    this.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