∀ {ι : Type u_2} {f : ι → Ordinal.{u_1} → Ordinal.{u_1}} {a : Ordinal.{u_1}}
  (motive : (∃ o, Ordinal.derivFamily f o = a) → Prop) (x : ∃ o, Ordinal.derivFamily f o = a),
  (∀ (w : Ordinal.{u_1}) (e : Ordinal.derivFamily f w = a), motive ⋯) → motive x