simp only [RegExp.is_nullable]
α : Type
⊢ RegExp.languageOf α RegExp.zero.delta =
    if RegExp.zero.is_nullable then RegExp.languageOf α RegExp.epsilon else RegExp.languageOf α RegExp.zero
α : Type
⊢ RegExp.languageOf α RegExp.zero.delta =
    if False then RegExp.languageOf α RegExp.epsilon else RegExp.languageOf α RegExp.zero