@@ -76,7 +76,7 @@ instance Additive (f (g a)) => Additive ((f :.: g) a)
7676-- | Product of groups, Functor style.
7777instance (Group (f a), Group (g a)) => Group ((f :*: g) a) where
7878 negateG (a :*: b) = negateG a :*: negateG b
79− (a :*: b) ~~ (c :*: d) = (a ~~ c :*: b ~~ d)
79+ (a :*: b) ~~ (c :*: d) = (a ~~ c) :*: (b ~~ d)
8080instance (Additive (f a), Additive (g a)) => Additive ((f :*: g) a)
8181
8282-- | Trivial group, Functor style