}

    @Override
    @CheckForNull
    public K ceiling(@ParametricNullness K e) {
      return map().ceilingKey(e);
    }

    @Override
    @CheckForNull
    public K higher(@ParametricNullness K e) {
      return map().higherKey(e);
    }

    @Override
    @CheckForNull
    public K pollFirst() {
      return keyOrNull(map().pollFirstEntry());
    }