∀ {R : Type u_2} [inst : Ring R] {α : Type u_1} (m : Multiset α) (f : α → R)
  (motive : (∃ a ∈ m, Polynomial.X - Polynomial.C (f a) = 0) → Prop)
  (x : ∃ a ∈ m, Polynomial.X - Polynomial.C (f a) = 0),
  (∀ (_a : α) (left : _a ∈ m) (ha : Polynomial.X - Polynomial.C (f _a) = 0), motive ⋯) → motive x