class Alt f where (<+>) :: f a -> f a -> f a
instance Alt Maybe where
  {-# NOINLINE (<+>) #-}
  l <+> r = l

{-# RULES
  "flatparse/reassoc-alt"  forall a. forall (l :: Maybe a) (m :: Maybe a) (r :: Maybe a). l <+> m <+> r = Nothing :: Maybe a
#-}

main = print $ (Just 1 <+> Just 2) <+> Just 3