case closure e _ =>
  simp only [RegExp.languageOf]
  simp only [RegExp.is_nullable]
  simp
α : Type
e : RegExp α
a_ih✝ :
  RegExp.languageOf α e.delta =
    if e.is_nullable then RegExp.languageOf α RegExp.epsilon else RegExp.languageOf α RegExp.zero
⊢ RegExp.languageOf α e.closure.delta =
    if e.closure.is_nullable then RegExp.languageOf α RegExp.epsilon else RegExp.languageOf α RegExp.zero
no goals