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

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

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