def _extend(self, newsub):
        '''
        Append a subclass (extension) after the base class. For parser internal use.
        '''
        current = self
        while hasattr(current, '_sub'):
            current = current._sub
        _set(current, '_sub', newsub)
        try:
            object.__delattr__(self, '_extra')
        except:
            pass
Append a subclass (extension) after the base class. For parser internal use.