[Haskell-cafe] instance Alternative (Either [a])

Hi guys! I'd like to implement the instance in the subject: instance Alternative (Either [a]) However the instance in Control.Monad.Trans.Error.Error gets in the way... I cannot use this one, it works only with Strings. My use case is that the Either handle in the Right a final value, or else in the Left a list of things "left to do" to compute the value. How can I do? Thanks! Corentin