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

[Richard Wordingham]

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. 

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

Richard,