\theoremindent adds space to left side when used with RTL languages

Question

Trying to use ntheorem's \theoremindent seems to indent the left side rather than the right side when used with polyglossia and right-to-left languages:

\documentclass{article}

\usepackage{ntheorem}
\theoremindent2cm
\theorembodyfont{\normalfont}
\theoremseparator{:}
\newtheorem{theorem}{משפט}

\usepackage{polyglossia}
\setdefaultlanguage[calendar=hebrew, numerals=arabic]{hebrew}
\setotherlanguage{english}
\newfontfamily\hebrewfont{David CLM}

\begin{document}
א א א א א א א א א א א א א א א א א א א א א א א א א א א א א א א א א א א א א א
\begin{theorem}
ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב ב
\end{theorem}
\end{document}

Is this fixable?

Also it seems the numbering and separators are displayed wrong.

Answer 1

The offending lines in ntheorem-hyper.sty (or ntheorem.sty, depending on your distribution) seem to be:

\newdimen\theoremindent
\global\theoremindent0cm
\newdimen\theorem@indent
%...
\advance\@totalleftmargin \theorem@indent
\parshape \@ne \@totalleftmargin \linewidth

So, unsurprisingly, ntheorem assumes margins are on the left. Here's a suggested micro-patch, partially due to @egreg:

\usepackage{ntheorem}
\usepackage{etoolbox}
\makeatletter
\@ifpackageloaded{polyglossia}{
\patchcmd{\@thm}
  {\advance\@totalleftmargin\theorem@indent}
  {\if@RTL\else\advance\@totalleftmargin\theorem@indent\fi}
  {}{}
}
\makeatother

This should be adaptable into a patch for ntheorem-hyper.sty itself.