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

[Werner LEMBERG]

>> Well, a reject pattern will be applied only when caches is reading.

By the way, this behaviour is undocumented in the `fonts.conf`
manpage.

>> To make it working, you may want to replace target="font" to
>> target="scan" in your config and run fc-cache -f to update.
> 
> Thanks, it works now.

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

```
<selectfont>
  <rejectfont>
    <pattern>
      <patelt>
        <more>
          <name>index</name>
          <int>65535</int>
        </more>
      </patelt>
    </pattern>
  </rejectfont>
</selectfont>
```

Any idea how this could be achieved?

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...


    Werner