Avoids having to call _update for each intermediate base.  Only
        works for class attr of type UpdateDict.

def _combine_attr_fast_update(self, attr, typ):
        '''Avoids having to call _update for each intermediate base.  Only
        works for class attr of type UpdateDict.

        '''
        values = dict(getattr(self, attr, {}))
        
        for base in self._class_data.bases:
            vals = dict(getattr(base, attr, {}))
            preserve_attr_data(vals, values)
            values = combine(vals, values)
            
        setattr(self, attr, typ(values))