(s : String) →
  (motive : Char → Option (s.Pos × ℕ) → Sort u_1) →
    (ch : Char) →
      (x : Option (s.Pos × ℕ)) →
        ((q : s.Pos) → (v : ℕ) → x = some (q, v) → motive 'x' (some (q, v))) →
          ((x_1 : Char) → (x_2 : Option (s.Pos × ℕ)) → x = x_2 → motive x_1 x_2) → motive ch x