sorry
case right
α : Type
inst✝ : DecidableEq α
a : α
r s : RegExp α
r_ih : ∀ (w : List α), w ∈ RegExp.languageOf α (RegExp.derivative a r) ↔ a :: w ∈ RegExp.languageOf α r
s_ih : ∀ (w : List α), w ∈ RegExp.languageOf α (RegExp.derivative a s) ↔ a :: w ∈ RegExp.languageOf α s
w xs : List α
a2_left : xs ∈ RegExp.languageOf α r
ys : List α
a3_left : ys ∈ RegExp.languageOf α s
a3_right : xs ++ ys = a :: w
c1 : [] ∈ RegExp.languageOf α r
⊢ [] ∈ RegExp.languageOf α s ∧ a :: w ++ [] = w
no goals