case a.zero.succ n ih =>
  rcases ih with ⟨⟨⟩, ⟨⟩, e⟩
  constructor <;> try constructor <;> try constructor <;> try assumption
  simp at *
a b : String
nab : ¬a = b
n : ℕ
ih : Ø ⋁ O +ₛ fromNat n =ₛ fromNat n +ₛ O
⊢ Ø ⋁ O +ₛ fromNat n↗ =ₛ fromNat n↗ +ₛ O
no goals