code: def err(self) -> Option[E]:
          
          return cast(Option[E], NONE) if self._is_ok else Option.Some(cast(E, self._val))
docs: Converts from :class:`Result` [T, E] to :class:`option.option_.Option` [E].
  
          Returns:
              :class:`Option` containing the error value if `self` is
              :meth:`Result.Err`, otherwise :data:`option.option_.NONE`.
  
          Examples:
              >>> Ok(1).err()
              NONE
              >>> Err(1).err()
              Some(1)