simp
case a
a b : String
n : ℕ
nab : ¬a = b
m : ℕ
⊢ Ø ⋁ fromNat n +ₛ _ b =ₛ _ b +ₛ fromNat n [b ≔ fromNat m]
case a
a b : String
n : ℕ
nab : ¬a = b
m : ℕ
⊢ Ø ⋁
    (Sinf_term.subst (fromNat n) b (fromNat m) +ₛ
        match String.decEq b b with
        | isTrue h => fromNat m
        | isFalse h => _ b) =ₛ
      (match String.decEq b b with
        | isTrue h => fromNat m
        | isFalse h => _ b) +ₛ
        Sinf_term.subst (fromNat n) b (fromNat m)