code: def extend(self, xs: Union['List[T]', typing.List[T]]) -> 'List[T]':  # type: ignore
          
          return type(self)(self.unbox() + List(xs).unbox())
docs: doufo.List.extend  
          Args:  
              `self`  
              `xs` (`Union['List[T]', typing.List[T]]`): Another List object or Typing.List  
          Returns:  
              extented `List` (`List[T]`)