[XeTeX] Diacritics on ligature components

[Peter Baker]

I'm working on a Latin-script font based on a medieval hand: it has lots 
of ligatures and contextual substitutions, and it uses (or will use) 
lots of combining diacritics. I'm developing with FontForge and testing 
with XeTeX.

Because combining marks (especially marks of abbreviation) can come on 
so many characters in a medieval Latin text, I'm trying to implement 
ligature marks for anchoring diacritics. ("Ligature marks" is a very 
unpleasant term to Google, by the way.) As far as I can tell, FontForge 
does this correctly: the output looks to be according to the 
specification, anyway.

What I haven't been able to figure out is how to input text in XeTeX (or 
any other program, for that matter) to test this. Documentation seems 
hard to come by (or perhaps is in languages where diacritics on 
ligatures are more common--ones I don't read). For example, I have a 
ligature "e_n" which a GSUB lookup substitutes for "en". Each component 
(the e and the n) has an anchor, and there are matching anchors on 
<acute> and <macron>. If I type "en<acute><macron>" then the two marks 
end up over the "e", on top of each other. If I type "e<acute>n<macron>" 
then the ligature never gets made.

I think I must be missing something obvious, but what? Thanks for any help.

Peter