simp only [equiv_language_of_closure]
α : Type
e : RegExp α
a_ih✝ : e.is_nullable ↔ [] ∈ RegExp.languageOf α e
⊢ e.closure.is_nullable ↔ [] ∈ RegExp.languageOf α e.closure
α : Type
e : RegExp α
a_ih✝ : e.is_nullable ↔ [] ∈ RegExp.languageOf α e
⊢ e.closure.is_nullable ↔ [] ∈ RegExp.languageOf α (RegExp.epsilon.union (e.concat e.closure))