∀ {s : String.Slice} (motive : Option s.Pos → Sort u_1) (pos : s.Pos)
  (h_1 : (pos_1 : s.Pos) → some pos = some pos_1 → motive (some pos_1)) (h_2 : some pos = none → motive none),
  (match hd : some pos with
    | some pos_1 => h_1 pos_1 hd
    | none => h_2 hd) =
    h_1 pos ⋯