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I am trying to modify a font by adding 'ccmp' basic ligature-type substitutions of the form:
sub a uniE151 by uni0061E151; # uniE151 has unicode value 0xE151 (PUA)
However, it doesn't seem to work except in Indesign CS6 and Mellel, not MSWord or TextEdit (on the Mac, by the way). However, if I do a substitution such as:
sub a uni02F9 by uni0061E151; # uni0279 has unicode value 0x02F9 (Non-PUA)
then it works as expected. I have tried this with several fonts and the same result is observed, it works in Indesign and Mellel but not in MS Word or TextEdit.
Is there something about using an PUA glyph in a substitution lookup that renders it ambiguous or illegal?