code: BaseRow getElement(int rank) {
  		int curRank = 0;
  		Iterator<Map.Entry<BaseRow, Collection<BaseRow>>> iter = treeMap.entrySet().iterator();
  		while (iter.hasNext()) {
  			Map.Entry<BaseRow, Collection<BaseRow>> entry = iter.next();
  			Collection<BaseRow> list = entry.getValue();
  
  			Iterator<BaseRow> listIter = list.iterator();
  			while (listIter.hasNext()) {
  				BaseRow elem = listIter.next();
  				curRank += 1;
  				if (curRank == rank) {
  					return elem;
  				}
  			}
  		}
  		return null;
  	}
docs: Gets record which rank is given value.
  
  @param rank rank value to search
  @return the record which rank is given value