+
+Note [SpecConstr for casts]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ data family T a :: *
+ data instance T Int = T Int
+
+ foo n = ...
+ where
+ go (T 0) = 0
+ go (T n) = go (T (n-1))
+
+The recursive call ends up looking like
+ go (T (I# ...) `cast` g)
+So we want to spot the construtor application inside the cast.
+That's why we have the Cast case in argToPat
+
+