∀ (s : String) (motive : (s.toList = [] ∨ ∃ l' b, s.toList = l'.concat b) → Prop)
  (x : s.toList = [] ∨ ∃ l' b, s.toList = l'.concat b),
  (∀ (h : s.toList = []), motive ⋯) → (∀ (cs : List Char) (c : Char) (h : s.toList = cs.concat c), motive ⋯) → motive x