code: public void initBackingIndexLoad(IOResourceProvider file, int pageId, int pageCount) {
  		if (idx != null) {
  			throw new IllegalStateException();
  		}
  		//8 byte page, 1 byte flag 
  		idx = new PagedUniqueLongLong(PAGE_TYPE.FREE_INDEX, file, pageId, 4, 8);
  		lastPage.set(pageCount-1);
  		iter = idx.iterator(1, Long.MAX_VALUE);//pageCount);
  	}
docs: Constructor for creating new index.
  @param file The file
  @param pageId The page ID of the root page
  @param pageCount Current number of pages