guava/src/com/google/common/collect/AbstractSortedSetMultimap.java
@CanIgnoreReturnValue
  @Override
  public SortedSet<V> removeAll(@Nullable Object key) {
    return (SortedSet<V>) super.removeAll(key);
  }
@CanIgnoreReturnValue
  @Override
  public SortedSet<V> removeAll(@Nullable Object key) {
    return (SortedSet<V>) super.removeAll(key);
  }
Removes all values associated with a given key. The returned collection is immutable.

<p>Because a {@code SortedSetMultimap} has unique sorted values for a given key, this method
returns a {@link SortedSet}, instead of the {@link Collection} specified in the {@link
Multimap} interface.