code: public DocId getParent(int n, BitSet deleted) throws IOException {
          return getBase().getParent(n, deleted);
      }
docs: Returns the <code>DocId</code> 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.
  @param deleted the documents that should be regarded as deleted.
  @return the <code>DocId</code> of <code>n</code>'s parent.
  @throws IOException if an error occurs while reading from the index.