biblatex 中的排序顺序

Question

在下面的例子中，我得到了第二个年份\printbibliography：

1960
1909a
1909b

是否有可能也改变 a/b ？

1960
1909b
1909a

\documentclass{article}
\usepackage[american]{babel}
\usepackage{csquotes}
\usepackage{hyperref}
\usepackage[sorting=nyt,sortcites,style=authoryear]{biblatex}
\addbibresource{biblatex-examples.bib}

\DeclareSortingTemplate{yntd}{
    \sort[direction=descending]{
        \field[strside=left,strwidth=4]{sortyear}
        \field[strside=left,strwidth=4]{year}
        \literal{9999}
    }
    \sort{
        \field{sortname}
        \field{author}
        \field{editor}
    }
    \sort[direction=descending]{
        \field{sorttitle}
        \field{title}
    }
}

\begin{document}
\cite{vangennep:related, vangennep:trans, vangennep}
\printbibliography
\newrefcontext[sorting=yntd]
\cite{vangennep:related, vangennep:trans, vangennep}
\printbibliography
\end{document}