* {@link MapNode} which additionally preserves user-supplied ordering. This node represents a data instance of
* a {@code list} with {@code ordered-by user;} substatement and a {@code key} definition.
*/
-public interface SystemMapNode extends MapNode, OrderingAware.System {
+public non-sealed interface SystemMapNode extends MapNode, OrderingAware.System {
@Override
default Class<SystemMapNode> contract() {
return SystemMapNode.class;