enumitem 忽略级别规范？

Question

考虑一下代码

\documentclass{article}
\usepackage{enumitem}
\newlist{longitemize}{itemize}{20}%
\setlist[longitemize,1]{label=\textbullet}%
\setlist[longitemize,2]{label=\normalfont \bfseries \textendash}%
\setlist[longitemize,3]{label=\textasteriskcentered}%
\setlist[longitemize,4]{label=\textperiodcentered}%
\setlist[longitemize,5]{label=\textbullet}%
\setlist[longitemize,6]{label=\normalfont \bfseries \textendash}%
\setlist[longitemize,7]{label=\textasteriskcentered}%
\setlist[longitemize,8]{label=\textperiodcentered}%
\setlist[longitemize,9]{label=\textbullet}%
\setlist[longitemize,10]{label=\normalfont \bfseries \textendash}%
\setlist[longitemize,11]{label=\textasteriskcentered}%
\setlist[longitemize,12]{label=\textperiodcentered}%
\setlist[longitemize,13]{label=\textbullet}%
\setlist[longitemize,14]{label=\normalfont \bfseries \textendash}%
\setlist[longitemize,15]{label=\textasteriskcentered}%
\setlist[longitemize,16]{label=\textperiodcentered}%
\setlist[longitemize,17]{label=\textbullet}%
\setlist[longitemize,18]{label=\normalfont \bfseries \textendash}%
\setlist[longitemize,19]{label=\textasteriskcentered}%
\setlist[longitemize,20]{label=\textperiodcentered}%

\begin{document}
\begin{longitemize}\item a
\begin{longitemize}\item a
\begin{longitemize}\item a
\begin{longitemize}\item a
\begin{longitemize}\item a
\begin{longitemize}\item a
\begin{longitemize}\item a
\begin{longitemize}\item a
\begin{longitemize}\item a
\begin{longitemize}\item a
\begin{longitemize}\item a
\begin{longitemize}\item a
\end{longitemize}
\end{longitemize}
\end{longitemize}
\end{longitemize}
\end{longitemize}
\end{longitemize}
\end{longitemize}
\end{longitemize}
\end{longitemize}
\end{longitemize}
\end{longitemize}
\end{longitemize}

\end{document}

我原以为这会让我深度嵌套列表，但我得到的是

This is pdfTeX, Version 3.14159265-2.6-1.40.18 (TeX Live 2017/Debian) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./foo.tex
LaTeX2e <2017-04-15>
Babel <3.18> and hyphenation patterns for 7 language(s) loaded.
(/usr/share/texlive/texmf-dist/tex/latex/base/article.cls
Document Class: article 2014/09/29 v1.4h Standard LaTeX document class
(/usr/share/texlive/texmf-dist/tex/latex/base/size10.clo))
(/usr/share/texlive/texmf-dist/tex/latex/enumitem/enumitem.sty) (./foo.aux)
(/usr/share/texlive/texmf-dist/tex/latex/base/omscmr.fd)

! LaTeX Error: Too deeply nested.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...

l.32 \begin{longitemize}\item
                              a
?

怎么回事？我该如何更深层次地嵌套项目符号列表？（我知道建议不要这样做。无论如何我都想这样做，以便能够pandoc编译 markdown 文件上的输出，这样我就可以在处理文档时在文档中看到它。）