使用 xparse 进行递归多重下标和上标

Question

如何使用 xparse 定义一个新命令，如下所示：

\NewDocumentCommand\tensorkor{ m >{\SplitList{,}}o  >{\SplitList{,}}o }
{
 ?????
}

我可以执行

$  \tensorkor{T}{^a,_b,_c,^r,^f}  $   

这应该导致像这样的代码

$  {{{{{T^a}_b}_c}^r}^f} $

这很棘手？我不明白ProcessList{#1}

Answer 1

\documentclass{article}
\usepackage{xparse}
\NewDocumentCommand{\tensorkor}{m >{\SplitList{,}}O{}}
 {\begingroup
  \mathsf{#1}%
  \newcommand\object{\vphantom{\mathsf{#1}}}%
  \ProcessList{#2}{\dotensorkor}%
  \endgroup}
\NewDocumentCommand{\dotensorkor}{m}
 {%
  {\object}#1%
 }
\begin{document}
$\tensorkor{T}[^a,_b,_c,^r,^f]$
\end{document}

Answer 2

不使用 xparse。(已编辑以提供)两个版本，取决于用户是否实际想要下标的下标（\recursa）或不想要（\recursb）：

\documentclass{article}
\usepackage{readarray}
\makeatletter
\newcounter{index}
\newcommand\recursa[1]{%
  \def\theresult{}%
  \getargsC{#1}%
  \setcounter{index}{0}%
  \whiledo{\value{index} < \narg}{%
    \addtocounter{index}{1}%
    \protected@edef\theresult{\bgroup\theresult\csname arg\roman{index}\endcsname\egroup}%
  }%
\theresult%
}
\newcommand\recursb[1]{%
  \def\theresult{}%
  \getargsC{#1}%
  \setcounter{index}{0}%
  \whiledo{\value{index} < \narg}{%
    \addtocounter{index}{1}%
    \protected@edef\theresult{\theresult\csname arg\roman{index}\endcsname{}}%
  }%
\theresult%
}
\makeatother
\parindent 0em\parskip 1em
\begin{document}
What the user asked for:\\
$  {{{{{T^a}_b}_c}^r}^f} $

Producing what the user asked for:\\
$\recursa{T ^a _b _c ^r ^f}$

Perhaps this is what the user really wanted:\\
$\recursb{T ^a _b _c ^r ^f}$

\end{document}