apply S_induction
a b : String
n : ℕ
nab : ¬a = b
⊢ Ø ⋁ ∀ₛ b ; fromNat n +ₛ _ b =ₛ _ b +ₛ fromNat n
case a
a b : String
n : ℕ
nab : ¬a = b
⊢ ∀ (n_1 : ℕ), Ø ⋁ fromNat n +ₛ _ b =ₛ _ b +ₛ fromNat n [b ≔ fromNat n_1]