@[reducible] protected def A.b : A → Nat :=
fun self => self.2
@[reducible] private def A.c : A → Nat :=
fun self => self.3
@[reducible] private def C.d : C → Nat :=
fun self => self.2
@[reducible] protected def C.e : C → Nat :=
fun self => self.3