# tag an equation

I want to give a name to an equation which is "the reverse of ن"

i'm using polyglossia when I write

    \documentclass[12pt,a4paper,openany]{book}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\usepackage{titlesec}
\usepackage{titletoc}
\usepackage{etoolbox}
\usepackage{multicol}
\usepackage{titleps}
%%%%%%%%%%%%%%%%%%%%%%%%%%
\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=1.15}
\usepackage{mathrsfs}
\usepackage{amsmath,amssymb,amsthm}
\usepackage{geometry}
%%%%%%%%%%%%%%%%%%%
\usetikzlibrary{arrows}
\usepgfplotslibrary{fillbetween}
\usetikzlibrary{patterns}
\usetikzlibrary{patterns,shapes.geometric}
%%%%%%%%%%%%%%%%%%%
\usepackage{fancyhdr}
\pagestyle{fancy}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\usepackage{polyglossia}
\setmainlanguage[numerals=maghrib]{arabic}
\setotherlanguage{english}
\newfontfamily\arabicfont[Script=Arabic,Scale=1.1,AutoFakeSlant=-0.03]{Amiri}
\setsansfont[Script=Arabic,Scale=1.1]{Amiri}

\begin{document}

$$\label{n}{\tag{\reflectbox{ن}}} a=b$$
\end{document}


it gives me many errors at end{equation}

the first is

undefined control sequence.
\bidiGscale@box...\fi\leavevmode\def\Gscale@y{#2}\se...


but if a delete \tag there is no error , how to de please to give this name to the equation?

Thank you

• You only provided an untestable fragment, I completed it to a document for testing but I get no error, please edit the example so it makes an error and show the error that you get (the form you show is not the format of a tex error message) Jul 23 '19 at 12:11
• @DavidCarlisle i edited the message Jul 23 '19 at 12:43
• The posted code does not produce the error you state it produces an undefined command error ! Undefined control sequence. <argument> ...(\kern \z@ \ignorespaces \eflectbox {ن}\unskip \@@italiccorr )... l.42  because of the typo \eflectbox instead of \reflectbox Jul 23 '19 at 12:46
• I get no error with \protect\reflectbox. The parentheses are reversed, though. Jul 23 '19 at 12:55
• this code is not the first , I reduce it for the question Jul 23 '19 at 13:03