*
* @param <T> Value type of Leaf entries
*/
-// // FIXME: 9.0.0: we really want to do a Set<@NonNull V> body(), but need to reconcile that with key-based lookup in
-// implementations -- and those are using only a Map internally.
-public interface SystemLeafSetNode<T> extends LeafSetNode<T>, OrderingAware.System {
+// FIXME: 9.0.0: we really want to do a Set<@NonNull V> body(), but need to reconcile that with key-based lookup in
+// implementations -- and those are using only a Map internally.
+public non-sealed interface SystemLeafSetNode<T> extends LeafSetNode<T>, OrderingAware.System {
@Override
@SuppressWarnings("rawtypes")
default Class<SystemLeafSetNode> contract() {