data JoinList m a = Empty
                  | Single m a
                  | Append m (JoinList m a) (JoinList m a)
    deriving (Eq, Show)

tag :: Monoid m => JoinList m a -> m
tag Empty                 = mempty
tag (Single x _)          = x
tag (Append x left right) = x

*JoinList Data.Monoid> tag (Append (Product 100) (Single (Product 10) "foo") (Single (Product 10) "bar"))
Product {getProduct = 100}

*JoinList Data.Monoid> tag (Empty :: JoinList (Product Integer) String)
Product {getProduct = 1}

*JoinList Data.Monoid> tag (Single (Product 5) "foo")
Product {getProduct = 5}

tag (Append x left right) = x  `mappend` (tag left) `mappend` (tag right)