sorry
α : Type
inst✝ : DecidableEq α
a : α
e : RegExp α
ih : ∀ (w : List α), w ∈ RegExp.languageOf α (RegExp.derivative a e) ↔ a :: w ∈ RegExp.languageOf α e
w xs : List α
a2_left : xs ∈ RegExp.languageOf α (RegExp.derivative a e)
ys : List (List α)
a3_left : ∀ r ∈ ys, r ∈ RegExp.languageOf α e
a3_right : xs ++ ys.join = w
⊢ w ∈ RegExp.languageOf α (RegExp.derivative a e)
no goals