[XeTeX] Low level XeTeX font features

[Will Robertson]

Hi

AAT-info.tex shows how you can use \XeTeXcountfeatures to loop through 
all of the available AAT features in a font and show which are selected 
by default.

Is there an analogous command for which features in a font are 
*currently* selected?

(For example, AAT-info.tex shows the same result whether the font 
chosen is "Big Caslon" or "Big Caslon:Letter Case=Small Capitals", 
whereas a hypothetical \XeTeXselectedfeatures command would not.)

Cheers,
Will