bug#36717: 25.3; greek.el: deprecated vowel+oxia combinations should be replaced with vowel+tonos counterparts

[Robert Alessi <alessi@robertalessi.net>]

On Thu, Jul 18, 2019 at 08:57:16PM +0200, Robert Pluim wrote:
> 
> But those methods that have tonos are for 'modern' greek, whereas the
> ones that have oxia are for classical greek, as Basil pointed out, so
> perhaps thereʼs no need to change anything (unless thereʼs an edict
> from the Unicode people that tonos must be used even when writing
> classical greek).

I was about to write that, as a classicist, if it were up to me I
wouldn't change anything myself.  So I would agree with Basil's
argument.  However, to take this one example, if one tries to use
Alexey Kryukov's beautiful Old Standard font with emacs and XeLaTeX or
LuaLaTeX, and activates the `ss06` feature which is supposed to
automatically make the distinction between initial and medial beta, he
will see that ὁ βάρβαρος typed with emacs succeeds while λάβρος fails,
just because of ά with oxia instead of ά with tonos.

Yesterday, I tried to get to the bottom of this: I cloned the source
files of Old Standard (https://github.com/akryukov/oldstand) and
edited the source file of the regular shape with FontForge.  I
included all of the seven letters with oxia that were missing in the
substitution rules and generated a new .otf file, but to no avail.
Actually, I discovered that even if one selects those letters,
FontForge automatically undoes this selection to select the
corresponding letters with tonos!  Here is the condition: as far as I
can tell, there is no way to get this kind of feature to work using
the letters with oxia. I myself consider this preposterous.

Then I came across digitalclassicist.org.  I modified my own greek.el
and got a Greek text perfectly typeset with all the required
substitutions.

In my opinion, this is a serious problem.  I will try to proceed
further on this line of inquiry.  An interesting question is: why
(unless I am mistaken) did FontForge deprecrate oxia in favor of
tonos?  I am asking just because I am experiencing this regression.
That said, I will take some time to go through
https://www.unicode.org/versions/ and report back if I find anything
of relevance.

Robert