simp only [RegExp.languageOf]
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 : List α
c1 : r.is_nullable
⊢ w ∈ RegExp.languageOf α ((RegExp.derivative a r).concat s) ↔ a :: w ∈ RegExp.languageOf α (r.concat s)
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 : List α
c1 : r.is_nullable
⊢ w ∈ {x | ∃ r_1 ∈ RegExp.languageOf α (RegExp.derivative a r), ∃ s_1 ∈ RegExp.languageOf α s, r_1 ++ s_1 = x} ↔
    a :: w ∈ {x | ∃ r_1 ∈ RegExp.languageOf α r, ∃ s_1 ∈ RegExp.languageOf α s, r_1 ++ s_1 = x}