simp only [RegExp.is_nullable]
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
⊢ {[]} = if e.closure.is_nullable then {[]} else ∅
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
⊢ {[]} = if True then {[]} else ∅