simp at ih_3
α : Type
L : Language α
s xs ys : Str α
a✝ : xs ∈ kleene_closure α L
ih_2 : ys ∈ L
ih_3 : xs ∈ ⋃ n, exp L n
⊢ xs ++ ys ∈ ⋃ n, exp L n
α : Type
L : Language α
s xs ys : Str α
a✝ : xs ∈ kleene_closure α L
ih_2 : ys ∈ L
ih_3 : ∃ i, xs ∈ exp L i
⊢ xs ++ ys ∈ ⋃ n, exp L n