}
        }
        return map().keySet().removeAll(toRemove);
      }
    }

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