diff --git a/groups.cabal b/groups.cabal
--- a/groups.cabal
+++ b/groups.cabal
@@ -1,6 +1,6 @@
 cabal-version:       2.4
 name:                groups
-version:             0.5
+version:             0.5.1
 synopsis:            Groups
 description:
   A group is a monoid with invertibility.
diff --git a/src/Data/Group.hs b/src/Data/Group.hs
--- a/src/Data/Group.hs
+++ b/src/Data/Group.hs
@@ -14,6 +14,7 @@
 import Data.Functor.Identity
 #endif
 #if MIN_VERSION_base(4,12,0)
+import Data.Functor.Contravariant (Op(Op))
 import GHC.Generics
 #endif
 
@@ -128,6 +129,8 @@
 instance Cyclic () where
   generator = ()
 
+instance Integral a => Cyclic (Sum a) where
+  generator = Sum 1
 #if MIN_VERSION_base(4,7,0)
 -- | Trivial group, Functor style.
 instance Group (Proxy x) where
@@ -168,6 +171,7 @@
 
 -- (:*:) and (:.:) exist since base-4.6.0.0 but the Monoid instances
 -- arrive in base-4.12.0.0.
+-- Also, contravariant was moved into base in this version.
 #if MIN_VERSION_base(4,12,0)
 -- | Product of groups, Functor style.
 instance (Group (f a), Group (g a)) => Group ((f :*: g) a) where
@@ -184,4 +188,10 @@
 instance (Abelian (f a), Abelian (g a)) => Abelian ((f :*: g) a)
 
 instance Abelian (f (g a)) => Abelian ((f :.: g) a)
+
+instance Group a => Group (Op a b) where
+  invert (Op f) = Op (invert f)
+  pow (Op f) n = Op (\e -> pow (f e) n)
+
+instance Abelian a => Abelian (Op a b)
 #endif