https://bugs.freedesktop.org/show_bug.cgi?id=58941

--- Comment #14 from Georg Duffner <[email protected]> ---
(In reply to comment #13)
> Suppose the default glyph for "a" in a font has a long tail, similar to
> Helvetica or Arimo/Liberation Sans. Suppose that the author has created two
> stylistic sets: "ss01" removes that tail from the a, and "ss02" makes it a
> one-story a. This would create a conflict if the user activated both at
> once, so this problem would have to be addressed. Perhaps, if the user tries
> to activate two conflicting features at once, there should be an error
> message.

This is not an issue for the rendering engine. It only has to cycle through the
lookups and apply one active GSUB rule after the other. It’s the font
designer’s task to make sure to prevent inconsistencies. 

In your case, there would be a feature ss01: substitute a by a.notail and ss02:
substitute a by a.onestory. In this case, the program will find an “a” in a
text, cycle through the lookups and find a match in ss01, so a gets replaced
with a.notail. Then it continues but won’t find a match in ss02 because now
there’s no longer an a but an a.notail for which no lookup exists in ss02. If
the font designer wished to get a match here too, he would add substitute
a.notail by a.onestory to his ss02 lookups. You see, the order in which the
lookups appear in the font is important (there’s no fixed order, one is
absolutely free to put ss02 before ss01 in their font), LO doesn’t have an
influence here and there’s no need for error messages. It’s more a question of
the font’s documentation.

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