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 cache the number of expected
+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}
-- Constants for the UnfWhen constructor