synchronized (mutex) {
        return Sets.equalsImpl(delegate(), o);
      }
    }

    @Override
    public boolean remove(@Nullable Object o) {
      synchronized (mutex) {
        return Maps.removeEntryImpl(delegate(), o);
      }
    }

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

        return Sets.equalsImpl(delegate(), o);
      }
    }

    @Override
    public boolean remove(@Nullable Object o) {
      synchronized (mutex) {
        return Maps.removeEntryImpl(delegate(), o);
      }
    }

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

   * @param o the object to remove from {@code c}
   * @return {@code true} if {@code c} was changed
   */
  static <K extends @Nullable Object, V extends @Nullable Object> boolean removeEntryImpl(
      Collection<Entry<K, V>> c, @Nullable Object o) {
    if (!(o instanceof Entry)) {
      return false;
    }
    return c.remove(unmodifiableEntry((Entry<?, ?>) o));
  }

   * @param o the object to remove from {@code c}
   * @return {@code true} if {@code c} was changed
   */
  static <K extends @Nullable Object, V extends @Nullable Object> boolean removeEntryImpl(
      Collection<Entry<K, V>> c, @Nullable Object o) {
    if (!(o instanceof Entry)) {
      return false;
    }
    return c.remove(unmodifiableEntry((Entry<?, ?>) o));
  }