simp only [RegExp.languageOf]
α : Type
r s : RegExp α
r_ih : r.is_nullable ↔ [] ∈ RegExp.languageOf α r
s_ih : s.is_nullable ↔ [] ∈ RegExp.languageOf α s
⊢ (r.concat s).is_nullable ↔ [] ∈ RegExp.languageOf α (r.concat s)
α : Type
r s : RegExp α
r_ih : r.is_nullable ↔ [] ∈ RegExp.languageOf α r
s_ih : s.is_nullable ↔ [] ∈ RegExp.languageOf α s
⊢ (r.concat s).is_nullable ↔ [] ∈ {x | ∃ r_1 ∈ RegExp.languageOf α r, ∃ s_1 ∈ RegExp.languageOf α s, r_1 ++ s_1 = x}