BidiMirrored property and ancient scripts (Was Re: Plain text custom fraction input)

[Eli Zaretskii]

> Date: Sat, 25 Jul 2015 08:44:22 +0100
> From: Richard Wordingham <richard.wordingham at ntlworld.com>
> 
> On Sat, 25 Jul 2015 10:14:58 +0300
> Eli Zaretskii <eliz at gnu.org> wrote:
> 
> > From: Richard Wordingham <richard.wordingham at ntlworld.com>
> > > Asmus Freytag <asmusf at ix.netcom.com> wrote:
> 
> > > > We do want "BidiMirrorred=No" to be honored; for example for the
> > > > arrows and the ornate parens. And we do not want that to be
> > > > overridden
>  
> > > And at present, that may be overridden in a right-to-left context!
> 
> > What do you mean by "overridden" in this context?  AFAIK, mirroring
> > indeed depends on context, but a character whose BidiMirrorred
> > property is No will _never_ be mirrored, according to the UBA.  There
> > are no overrides for that property, AFAIK.
> 
> Reread the Bidi algorithm, especially
> http://www.unicode.org/reports/tr9/#L4 and
> http://www.unicode.org/reports/tr9/#HL6.
> 
> In principle, I could have a higher-level protocol that mirrors lamedh
> on Wednesdays, but I must follow the rules for parentheses. 

I don't see how this is related.  What HL6 describes is something that
should make sense.  For example, Emacs uses '/' as a kind of
"mirrored" '\', when it needs to indicate that a line in an R2L
paragraph is continued on the next screen line.

By contrast, indiscriminately mirroring random characters that don't
really have mirrored glyphs, in the context of modern scripts, doesn't
make any sense, IMO, so it should never be done.

> It's part of the tendency to write specifications as 'Do what you want,
> but we recommend...'.  It eliminates non-compliances without increasing
> compatibility.

Just say no.