[Fontconfig] how to modify the `variable` property?

[Werner LEMBERG]

>> By the way, this behaviour is undocumented in the `fonts.conf`
>> manpage.
> 
> Sure. This can be definitely improved.

Great!  It is probably sufficient to mention that `<acceptfont>` and
`<rejectfont>` are only influenced by `<match target="scan">` and not
by the targets 'font' and 'pattern'.

By the way, why is that so?  It looks like an artificial limitation,
and as my real-world example shows it can cause problems.

> that said:
> 
> --- quoted from doc
> Fonts matched by an rejectfont element are "blacklisted"; such fonts are
> excluded from the set of fonts used to resolve list and match requests
> --- end
> 
> And "font" target is applied to the result on matching. So it may not
> be that hard to imagine matching will be processed after filtering by
> acceptfont/rejectfont.

Believe me, it is that hard to imagine :-)

>> I answered too quickly.  It apparently doesn't work for LilyPond,
>> which uses an application-specific Fontconfig configuration file.  In
>> other words, I need a method to reject fonts with index > 0xFFFF that
>> doesn't need rescanning of all fonts in the OS, something like
> 
> They don't have a sort of 50-user.conf loaded?

No.  LilyPond uses the fonts from the OS but can't handle Variation
Fonts – for the default output backend it has to embed fonts in an
intermediate PS file, and PostScript doesn't support VFs.

>> Any idea how this could be achieved?
> 
> Unfortunately not because such conditions can't be represented in
> FcPattern.

In other words, there is no solution right now, correct?

>> If it weren't for backward compatibility I would simply ask to add a
>> new property `namedinstance` that gets set as soon as the font index
>> is larger than 0xFFFF...
> 
> That would be possible. please file an rfe to
> https://gitlab.freedesktop.org/fontconfig/fontconfig/-/issues

Done:

  https://gitlab.freedesktop.org/fontconfig/fontconfig/-/issues/362


    Werner