[HarfBuzz] About GSUB backtrack, lookahead glyph for first and last character

[Leone Kim]

Hi, my name is leone and
these days I'm working on a small project on my own
which make cursive english font using GSUB - chaining contextual substitute
(coverage based) (which will be applied on my android phone)

the problem is I can't figure it out how to organize backtrack, lookahead
Glyph coverage for the first and last character (For example 'x' & 'g' in
"xyzabcdefg")
What should I set backtrack glyph for 'x' and lookahead glyph for 'g'?

every character i designed has
1 Ini (which has no previous latin glyph, but has latin glyph after it)
1 Fin (which has latin glypf in front of it, no latin glyph after itself)
26 Mid (which has latin glyph both front and end of it)

abcd   <-- b and c should display as Mid form,  a as Ini form, and d as Fin
form.

I tried to figure out how harfbuzz engine process GSUB table but failed (
T_T )
Please tell me how I should organize backtrack, lookahead Glyph coverage
for Ini and Fin. Thank you

sorry If I wrote this in any bad manner and for my bad english.
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