Since 0.9.7HarfBuzz.face_t to work upon
The index of the lookup to query
The sequence of glyphs to query for substitution
The length of the glyph sequence
HarfBuzz.bool_t indicating whether pre-/post-context are disallowed in substitutions
true if a substitution would be triggered, false otherwise
Tests whether a specified lookup in the specified face would trigger a substitution on the given glyph sequence.