code: def closeGlyphsOverGSUB(gsub, glyphs):
      
      subsetter = subset.Subsetter()
      subsetter.glyphs = glyphs
      gsub.closure_glyphs(subsetter)
docs: Use the FontTools subsetter to perform a closure over the GSUB table
      given the initial `glyphs` (set of glyph names, str). Update the set
      in-place adding all the glyph names that can be reached via GSUB
      substitutions from this initial set.