public GroupingOptions<G> setGroupPruner(final Predicate<ItemGroup<T, G>> groupPruner) {
            this.groupPruner = groupPruner;
            triggerChange(ChangeType.COMPLETE);
            return this;
        }
Sets the  minimum number of groups on which grouping is really activated
main/src/main/java/cgeo/geocaching/ui/SimpleItemListModel.java