simp only [Set.subset_def]
case a
α : Type
L : Language α
⊢ kleene_closure α L ⊆ {[]} ∪ concat L (kleene_closure α L)
case a
α : Type
L : Language α
⊢ ∀ x ∈ kleene_closure α L, x ∈ {[]} ∪ concat L (kleene_closure α L)