case char a =>
  simp only [RegExp.is_nullable]
  simp only [RegExp.languageOf]
  simp
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
no goals