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

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

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