refine Set.eq_univ_of_forall $ fun n ↦ sumset_contains_n hA hB ?_
A B : Set ℕ
n : ℕ
hA : 0 ∈ A
hB : 0 ∈ B
hAB : 1 ≤ schnirelmannDensity A + schnirelmannDensity B
⊢ A + B = Set.univ
A B : Set ℕ
n✝ : ℕ
hA : 0 ∈ A
hB : 0 ∈ B
hAB : 1 ≤ schnirelmannDensity A + schnirelmannDensity B
n : ℕ
⊢ n ≤ countelements A n + countelements B n