case _ a1_left_left a1_left_right =>
  simp only [a1_left_left]
  simp only [a1_left_right]
  simp
  simp only [a2_right]
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 : List α
a2_right : cs = []
a1_left_left : r.is_nullable
a1_left_right : s.is_nullable
⊢ ∃ r_1, (r.is_nullable ∧ r_1 = []) ∧ ∃ s_1, (s.is_nullable ∧ s_1 = []) ∧ r_1 ++ s_1 = cs
no goals