[luatex] \predisplaydirection from etex in luatex has no effect

[Vafa Khalighi]

I now want to say why \predisplaydirection is neeed. As I said in my last
email, there are RTL scripts that do maths LTR but the place of \eqno and
\leqno is interchanged. Assuming \predisplaydirection does not exist, one
can reverse \eqno and \leqno is two ways and considering latex, this works
for equation environment.

I am describing these two aproaches based on TeX--XeT but it can be done
similarly in luatex.

The first approach would be to say \eveydisplay{\aftergroup\beginR}

the second would be something like

\let\@equation\equation
\let\@endequation\endequation

\def\equation{\beginR\@equation}
\def\endequation{\@endequation\endR}

Both of these approach is not good. In implementing bidi package I have
played with these for hours and hours and I know that they are not going to
give satisfactory results. THe main proplem is that these two approaches is
that it increases the space as much as a blank paragraph between
equations.

So it makes sense to have a primitive like \predisplaydirection that one can
achieve this.

-- 
Best wishes,
Vafa Khalighi
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