code: @Override protected AutoBuffer writeAll_impl(AutoBuffer ab) {
      for (Key<CompressedTree>[] ks : _output._treeKeys)
        for (Key<CompressedTree> k : ks)
          ab.putKey(k);
      for (Key<CompressedTree>[] ks : _output._treeKeysAux)
        for (Key<CompressedTree> k : ks)
          ab.putKey(k);
      return super.writeAll_impl(ab);
    }
docs: Write out K/V pairs