Has anyone ever implemented GSUB type 8 lookups? Yannis Haralambous' book tells me that it was specifically designed for Urdu but I'm not sure I believe him, and I don't know of any Urdu fonts that use it. Does any font editing software even support it? (FontForge apparently crashes when it sees one
.) The only examples of it I've found on github
are AFDKO test files.
More to the point, can't reverse contextual substitutions always be rewritten in terms of forward contextual substitutions? What is the difference between
rsub foo wibble' by wibble.alt;
sub wibble' foo by wibble.alt;