apply Exists.intro xs
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
⊢ ∃ s ∈ exp L n, ∃ t ∈ L, s ++ t = xs ++ ys
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 ∈ exp L n ∧ ∃ t ∈ L, xs ++ t = xs ++ ys