simp only [concat]
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
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 ∈ {x | ∃ s ∈ exp L n, ∃ t ∈ L, s ++ t = x}