* <li>{@link Qualified}, which also holds a string prefix available via {@link Qualified#getPrefix()}.</li>
* </ol>
*/
-// FIXME: sealed to allow Qualified and Unqualified only when we have JDK17+
@NonNullByDefault
-public abstract class UnresolvedQName extends AbstractQName {
+public abstract sealed class UnresolvedQName extends AbstractQName {
/**
* An unresolved, qualified {@link QName}. It is guaranteed to hold a valid {@link #getLocalName()} bound to a
* namespace identified through a prefix string, but remains unresolved. A resolved {@link QName} can be obtained