code: def _validateClasses(classes):
      
      glyphToClass = {}
      for className, glyphList in classes.items():
          for glyphName in glyphList:
              if glyphName not in glyphToClass:
                  glyphToClass[glyphName] = set()
              glyphToClass[glyphName].add(className)
      for glyphName, groupList in glyphToClass.items():
          if len(groupList) > 1:
              raise ExtractorError("Kerning classes are in an conflicting state.")
docs: Check to make sure that a glyph is not part of more than
      one class. If this is found, an ExtractorError is raised.