simp only [exp]
α : Type
L : Language α
s xs ys : Str α
a✝ : xs ∈ kleene_closure α L
ih_2 : ys ∈ L
n : ℕ
a2 : xs ∈ exp L n
⊢ xs ++ ys ∈ exp L (n + 1)
α : Type
L : Language α
s xs ys : Str α
a✝ : xs ∈ kleene_closure α L
ih_2 : ys ∈ L
n : ℕ
a2 : xs ∈ exp L n
⊢ xs ++ ys ∈ concat (exp L n) L