code: def filtered(self, allowed):
          
          kws = {k:v for k,v in self.kwargs.items() if k in allowed}
          return self.__class__(key=self.key,
                                allowed_keywords=self.allowed_keywords,
                                merge_keywords=self.merge_keywords, **kws)
docs: Return a new Options object that is filtered by the specified
          list of keys. Mutating self.kwargs to filter is unsafe due to
          the option expansion that occurs on initialization.