code: final protected AbstractIndexPage getPrevLeafPage(AbstractIndexPage indexPage) {
  		int pos = getPagePosition(indexPage);
  		if (pos > 0) {
  			AbstractIndexPage page = getPageByPos(pos-1);
  			return page.getLastLeafPage();
  		}
  		if (getParent() == null) {
  			return null;
  		}
  		//TODO we really should return the last leaf page of the previous inner page.
  		//But if they get merged, then we have to shift minimal values, which is
  		//complicated. For now we ignore this case, hoping that it doesn't matter much.
  		return null;
  	}
docs: Returns only LEAF pages.
  TODO for now this ignores leafPages on a previous inner node. It returns only leaf pages
  from the current node.
  @param indexPage
  @return The previous leaf page or null, if the given page is the first page.