public void visit(GroupVisitor visitor) {
        if (isHierarchical()) {
            distribution.visitGroups(group -> {
                if (group.isLeafGroup()) {
                    return visitor.visitGroup(group);
                }

                return true;
            });
        }
    }
Invoke the visitor for each leaf group of an implied group.  If the group is non-hierarchical
(flat), the visitor will not be invoked.
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/HierarchicalGroupVisiting.java