+ $dfCList :: forall a. C a => C [a] -- Arity 2!
+ $dfCList = /\a.\d. $copList {a} d |> co
+
+ $copList :: forall a. C a => [a] -> Int -- Arity 2!
+ $copList = /\a.\d.\xs. op {a} d (head xs)
+
+Now we might encounter (op (dfCList {ty} d) a1 a2)
+and we want the (op (dfList {ty} d)) rule to fire, because $dfCList
+has all its arguments, even though its (value) arity is 2. That's
+why we record the number of expected arguments in the DFunUnfolding.
+
+Note that although it's an Arity, it's most convenient for it to give
+the *total* number of arguments, both type and value. See the use
+site in exprIsConApp_maybe.
+
+\begin{code}