[XeTeX] How to get _third_ stylistic alternate?

[Marcin Woliński]

Will Robertson wrote:
> I wonder if typesetting a letter with increasing 'salt' values and  
> inspecting the resulting box would (a) work reliably (I'd guess not,  
> sometimes), and (b) be useful for users to inspect the features of  
> their fonts.

IMHO it would be very usefull, especially for the ‘aalt’ feature.  In
the “OpenType Layout tag
registry” (http://www.microsoft.com/typography/otspec/features_ae.htm#aalt) the following user interface suggestion is given:

“The application should indicate to the user which glyphs in the user's
document have alternative forms (i.e which are in the coverage table for
aalt). When the user selects one of those glyphs and applies the aalt
feature, an application could display the forms sequentially in context,
or present a palette showing all the forms at once, or give the user a
choice between these approaches.”

So enumeration is definitely the intended use for ‘aalt’.  And it seems
a wise idea: we should have a way to explore the font from within XeTeX.
Imagine the OpenTypeinfo.tex extended with information on individual
characters…
Unfortunately I don't know how much work would that need.  Which
part/library is responsible for implementing features?

Of course we could also use some stand-alone tool for that but do we
have one (a free one)?  Fontforge comes to mind but that is probably an
overkill just to check what glyphs are in the font.

BTW, do I understand correctly that the only OT features with numeric
argument in XeTeX are  “alternate substitution” features where in the
OTspec parlance the aplication gets a group of GIDs as a result of a
lookup?

Best,
Marcin