. rw [Language.concat]
  intro ⟨u,v, h1, h2, h3⟩
  simp [Language.epsilon, Membership.mem] at h1
  simp [h3, h1, Word.epsilon]
  exact h2
case h.a.mp
α : Type u_1
L : Language α
w : Word α
⊢ (epsilon ∘ₗ L) w → L w

case h.a.mpr
α : Type u_1
L : Language α
w : Word α
⊢ L w → (epsilon ∘ₗ L) w
case h.a.mpr
α : Type u_1
L : Language α
w : Word α
⊢ L w → (epsilon ∘ₗ L) w