code: private void updateFirstKey(PageModificationContext context) {
          BTreePage page = context.getPageWrapper().getPage();
          assert page.getConfig().isIndexPage() : "expected index page";
          if (page.getCount() > 0) {
              page.setFirstKey(generateMinKey());
          }
      }
docs: Handles a special case for Index Nodes. Since each Page Pointer points to a Page which contains all Keys equal to
  or larger than their Pointer Key (but smaller than the next Pointer Key in the Page's parent), the first key in that
  Page must be the minimum possible key, otherwise we will not be able to insert any values smaller than this Pointer Key,
  but which would have to go in that Page.
  
  @param context Processing Context.