[XeTeX] Access All Alternates <aalt> feature

[David J. Perry]

Does XeTeX provide any way to access this feature?  I have looked 
through the Fontspec manual, with particular attention to §10.7ff, but 
found nothing.

The designer of the font I am testing put some substitutions under this 
feature.  I'm not sure why, since he could just as well have used a 
stylistic set or stylistic alternates (salt) since there is only one 
substitution per character.  But it's what I have to work with.

Thanks - David