code: public DocId getParentDocId(int n) throws IOException {
          int i = readerIndex(n);
          DocId result = DocId.NULL;
          if(i >= 0 && starts[i] >= 0 && (n - starts[i]) >= 0) {
             DocId id = subReaders[i].getParent(n - starts[i]);
             result  =  id.applyOffset(starts[i]);
          }
          return result;
      }
docs: Returns the DocId of the parent of <code>n</code> or {@link DocId#NULL}
  if <code>n</code> does not have a parent (<code>n</code> is the root
  node).
  
  @param n the document number.
  @return the DocId of <code>n</code>'s parent.
  @throws IOException if an error occurs while reading from the index.