import java.util.SortedSet;
import javax.annotation.CheckForNull;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A {@code SetMultimap} whose set of values for a given key are kept sorted; that is, they comprise
 * a {@link SortedSet}. It cannot hold duplicate key-value pairs; adding a key-value pair that's
 * already in the multimap has no effect. This interface does not specify the ordering of the