[Fontconfig] Problem when <alias> is followed by more than one <family>

[lolilolicon]

Hi, I need a little help understanding a <alias> config.

The font "Helvetica" is not installed on my system, but one called
"HelveticaNeueLT Pro 55 Roman" is; I like the later so I want to prepend
it before "Helvetica" and "Arial".  I find that <alias> works curiously
with more than one <family> being matched.

In ~/.fonts.conf, I have:

  <alias binding="same">
    <family>Helvetica</family>
    <family>Arial</family>
    <prefer>
      <family>HelveticaNeueLT Pro 55 Roman</family>
    </prefer>
  </alias>

then Arial is not aliased corretly:

  $ fc-match Helvetica
  HelveticaNeueLTPro-Roman.otf: "HelveticaNeueLT Pro 55 Roman" "55 Roman"
  $ fc-match Arial
  arial.ttf: "Arial" "Normal"

And if I put "Arial" at first, instead:

  <alias binding="same">
    <family>Arial</family>
    <family>Helvetica</family>
    <prefer>
      <family>HelveticaNeueLT Pro 55 Roman</family>
    </prefer>
  </alias>

then Helvetica is not aliased corretly:

  $ fc-match Helvetica
  n019003l.pfb: "Nimbus Sans L" "Regular"
  $ fc-match Arial
  HelveticaNeueLTPro-Roman.otf: "HelveticaNeueLT Pro 55 Roman" "55 Roman"

Here, "Nimbus Sans L" is from 30-metric-aliases.conf:

  <alias binding="same">
    <family>Helvetica</family>
    <accept>
      <family>Nimbus Sans L</family>
    </accept>
  </alias>

If I use separate <alias> for each family:

  <alias binding="same">
    <family>Helvetica</family>
    <prefer>
      <family>HelveticaNeueLT Pro 55 Roman</family>
    </prefer>
  </alias>
  <alias binding="same">
    <family>Arial</family>
    <prefer>
      <family>HelveticaNeueLT Pro 55 Roman</family>
    </prefer>
  </alias>

then the result is as expected:

  $ fc-match Helvetica
  HelveticaNeueLTPro-Roman.otf: "HelveticaNeueLT Pro 55 Roman" "55 Roman"
  $ fc-match Arial
  HelveticaNeueLTPro-Roman.otf: "HelveticaNeueLT Pro 55 Roman" "55 Roman"

Can you help me understand what is happening?