rw [String.decEq_refl a] at *
a : String
n : ℕ
ih :
  Ø ⋁
    ¬ₛ
      (match String.decEq a a with
          | isTrue h => fromNat n
          | isFalse h => _ a)↗ =ₛ
        O
⊢ Ø ⋁
    ¬ₛ
      (match String.decEq a a with
          | isTrue h => fromNat n↗
          | isFalse h => _ a)↗ =ₛ
        O
a : String
n : ℕ
ih :
  Ø ⋁
    ¬ₛ
      (match isTrue (_ : a = a) with
          | isTrue h => fromNat n
          | isFalse h => _ a)↗ =ₛ
        O
⊢ Ø ⋁
    ¬ₛ
      (match isTrue (_ : a = a) with
          | isTrue h => fromNat n↗
          | isFalse h => _ a)↗ =ₛ
        O