code: def _to_ufo_kerning(self, ufo, kerning_data):
      
  
      warning_msg = "Non-existent glyph class %s found in kerning rules."
  
      for left, pairs in kerning_data.items():
          match = re.match(r"@MMK_L_(.+)", left)
          left_is_class = bool(match)
          if left_is_class:
              left = "public.kern1.%s" % match.group(1)
              if left not in ufo.groups:
                  self.logger.warning(warning_msg % left)
          for right, kerning_val in pairs.items():
              match = re.match(r"@MMK_R_(.+)", right)
              right_is_class = bool(match)
              if right_is_class:
                  right = "public.kern2.%s" % match.group(1)
                  if right not in ufo.groups:
                      self.logger.warning(warning_msg % right)
              ufo.kerning[left, right] = kerning_val
docs: Add .glyphs kerning to an UFO.