doufo.List.extend  
        Args:  
            `self`  
            `xs` (`Union['List[T]', typing.List[T]]`): Another List object or Typing.List  
        Returns:  
            extented `List` (`List[T]`)

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