∀ (x : String.Legacy.Iterator) (motive : (∃ l r, String.Legacy.Iterator.ValidFor l r x) → Prop)
  (x_1 : ∃ l r, String.Legacy.Iterator.ValidFor l r x),
  (∀ (l r : List Char) (h : String.Legacy.Iterator.ValidFor l r x), motive ⋯) → motive x_1