∀ {M : Type u_2} {ι : Type u_1} {t : Finset ι} {f : ι → M} (_x : M) (motive : (∃ a ∈ t.val, f a = _x) → Prop)
  (x : ∃ a ∈ t.val, f a = _x), (∀ (i : ι) (hi : i ∈ t.val) (hix : f i = _x), motive ⋯) → motive x