Problem with theorem's numbering when using Arabi package

Question

When I use a theorem style environment with Arabi package, the numbering become corrupted just after the numbers with two digits start.

Here an example. The number of the last one should be 10 but it shows 01.

\documentclass{article}
\usepackage{amssymb, amsthm}
\usepackage[utf8]{inputenc}
\usepackage[LFE,LAE,OT1]{fontenc}
\usepackage[farsi,english,arabic]{babel}

\newtheorem{thm}{Theorem}
\theoremstyle{definition}
\newtheorem{exer}[thm]{تمرین}
\theoremstyle{plain}

\begin{document}
\selectlanguage{farsi}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\end{document}

Welcome to TeX.SX! A tip: If you indent lines by 4 spaces, they'll be marked as a code sample. You can also highlight the code and click the "code" button (with "{}" on it). — Jesse, Oct 10 '14 at 08:13

score 4 · Accepted Answer · edited Apr 13 '17 at 12:34

The number is of printed backwards, so we want to reverse it:

\documentclass{article}
\usepackage{amssymb, amsthm}
\usepackage[utf8]{inputenc}
\usepackage[LFE,LAE,OT1]{fontenc}
\usepackage[farsi,english,arabic]{babel}

\usepackage{xparse}
\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\revarabic}{m}
 {
  \tl_reverse:f { \arabic{#1} }
 }
\cs_generate_variant:Nn \tl_reverse:n { f }
\ExplSyntaxOff

\newtheorem{thm}{Theorem}
\theoremstyle{definition}
\newtheorem{exer}[thm]{تمرین}
\theoremstyle{plain}
\renewcommand\thethm{\revarabic{thm}}

\begin{document}
\selectlanguage{farsi}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\end{document}

enter image description here

Here is a version that doesn't use expl3; the macro is the same as in this answer of mine

\documentclass{article}
\usepackage{amssymb, amsthm}
\usepackage[utf8]{inputenc}
\usepackage[LFE,LAE,OT1]{fontenc}
\usepackage[farsi,english,arabic]{babel}

\makeatletter
\def\revarabic#1{%
  \expandafter\num@reverse\expandafter{\romannumeral-`Q\arabic{#1}}%
}
\def\num@reverse#1{\num@rev#1\num@rev@a\num@rev@b}
\def\num@rev#1#2\num@rev@a#3\num@rev@b{%
  \if\relax\detokenize{#2}\relax
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
  {#1#3}%
  {\num@rev#2\num@rev@a#1#3\num@rev@b}%
}
\makeatother


\newtheorem{thm}{Theorem}
\theoremstyle{definition}
\newtheorem{exer}[thm]{تمرین}
\theoremstyle{plain}
\renewcommand\thethm{\revarabic{thm}}

\begin{document}
\selectlanguage{farsi}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\end{exer}
\begin{exer}\label{x}\end{exer}

\ref{x}
\end{document}

I added a \label to show the reference is correct.

when I compile the file I get the following error: ERROR: Undefined control sequence.
--- TeX said --- l.13 \cs_generate_variant:Nn \tl_reverse:n { f } — Name, Oct 10 '14 at 10:56
@Name You have an outdated TeX system. I'll add later a version independent on expl3, but you should upgrade your TeX distribution anyway. — egreg, Oct 10 '14 at 10:59
I would be really grateful if you could provide a version independent form expl3. — Name, Oct 10 '14 at 11:09

Problem with theorem's numbering when using Arabi package

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