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