cases a2_left
α : Type
r s : RegExp α
r_ih : RegExp.languageOf α r.delta = if r.is_nullable then {[]} else ∅
s_ih : RegExp.languageOf α s.delta = if s.is_nullable then {[]} else ∅
cs xs : List α
a2_left : r.is_nullable ∧ xs = []
a2_right : ∃ s_1, (s.is_nullable ∧ s_1 = []) ∧ xs ++ s_1 = cs
⊢ (r.is_nullable ∧ s.is_nullable) ∧ cs = []
case intro
α : Type
r s : RegExp α
r_ih : RegExp.languageOf α r.delta = if r.is_nullable then {[]} else ∅
s_ih : RegExp.languageOf α s.delta = if s.is_nullable then {[]} else ∅
cs xs : List α
a2_right : ∃ s_1, (s.is_nullable ∧ s_1 = []) ∧ xs ++ s_1 = cs
left✝ : r.is_nullable
right✝ : xs = []
⊢ (r.is_nullable ∧ s.is_nullable) ∧ cs = []