synchronized (mutex) {
        return delegate().remove(o);
      }
    }

    @Override
    public boolean removeAll(Collection<?> c) {
      synchronized (mutex) {
        return delegate().removeAll(c);
      }
    }

    @Override
    public boolean retainAll(Collection<?> c) {
      synchronized (mutex) {
        return delegate().retainAll(c);
      }

      synchronized (mutex) {
        return delegate().remove(o);
      }
    }

    @Override
    public boolean removeAll(Collection<?> c) {
      synchronized (mutex) {
        return delegate().removeAll(c);
      }
    }

    @Override
    public boolean retainAll(Collection<?> c) {
      synchronized (mutex) {
        return delegate().retainAll(c);
      }

        Set<ApiChange> seenApiChanges = (Set<ApiChange>) context.getUserData().get("seenApiChanges");
        Set<ApiChange> left = new HashSet<>(acceptedApiChanges);
        left.removeAll(seenApiChanges);
        if (!left.isEmpty()) {
            String formattedLeft = CollectionUtils.join("\n", left);

 * already in the multimap has no effect. See the {@link Multimap} documentation for information
 * common to all multimaps.
 *
 * <p>The {@link #get}, {@link #removeAll}, and {@link #replaceValues} methods each return a {@link
 * Set} of values, while {@link #entries} returns a {@code Set} of map entries. Though the method

 * already in the multimap has no effect. See the {@link Multimap} documentation for information
 * common to all multimaps.
 *
 * <p>The {@link #get}, {@link #removeAll}, and {@link #replaceValues} methods each return a {@link
 * Set} of values, while {@link #entries} returns a {@code Set} of map entries. Though the method

            return true;
          }
        }
        return false;
      }
    }

    @Override
    public boolean removeAll(Collection<?> c) {
      try {
        return super.removeAll(checkNotNull(c));
      } catch (UnsupportedOperationException e) {
        Set<K> toRemove = newHashSet();
        for (Entry<K, V> entry : map().entrySet()) {

            return true;
          }
        }
        return false;
      }
    }

    @Override
    public boolean removeAll(Collection<?> c) {
      try {
        return super.removeAll(checkNotNull(c));
      } catch (UnsupportedOperationException e) {
        Set<K> toRemove = newHashSet();
        for (Entry<K, V> entry : map().entrySet()) {

public final class Sets {
  private Sets() {}

  /**
   * {@link AbstractSet} substitute without the potentially-quadratic {@code removeAll}
   * implementation.
   */
  abstract static class ImprovedAbstractSet<E extends @Nullable Object> extends AbstractSet<E> {
    @Override
    public boolean removeAll(Collection<?> c) {
      return removeAllImpl(this, c);
    }

    @Override
    public boolean retainAll(Collection<?> c) {

      return get((K) key).remove(value);
    }

    @SuppressWarnings("unchecked")
    @Override
    public Collection<V2> removeAll(@CheckForNull Object key) {
      return transform((K) key, fromMultimap.removeAll(key));
    }

    @Override
    public Collection<V2> replaceValues(@ParametricNullness K key, Iterable<? extends V2> values) {

    return remove(element, 1) > 0;
  }

  /**
   * A sensible definition of {@link #removeAll} in terms of the {@code removeAll} method of {@link
   * #elementSet}. If you override {@link #elementSet}, you may wish to override {@link #removeAll}
   * to forward to this implementation.
   *
   * @since 7.0
   */
  @Override