rw [← hca, ← hcb] at hc
A B : Set ℕ
n : ℕ
hA : 0 ∈ A
hB : 0 ∈ B
hc : n ≤ countelements A n + countelements B n
h : n ∉ A + B
hnA : n ∉ A
hnB : n ∉ B
hca : countelements A (n - 1) = countelements A n
hcb : countelements B (n - 1) = countelements B n
hn1 : n > 0
lem1 : (filter (fun x => x ∈ {n} - B) (Ioo 0 n)).card = countelements B (n - 1)
⊢ (filter (fun x => x ∈ A) (Ioo 0 n) ∩ filter (fun x => x ∈ {n} - B) (Ioo 0 n)).Nonempty
A B : Set ℕ
n : ℕ
hA : 0 ∈ A
hB : 0 ∈ B
hc : n ≤ countelements A (n - 1) + countelements B (n - 1)
h : n ∉ A + B
hnA : n ∉ A
hnB : n ∉ B
hca : countelements A (n - 1) = countelements A n
hcb : countelements B (n - 1) = countelements B n
hn1 : n > 0
lem1 : (filter (fun x => x ∈ {n} - B) (Ioo 0 n)).card = countelements B (n - 1)
⊢ (filter (fun x => x ∈ A) (Ioo 0 n) ∩ filter (fun x => x ∈ {n} - B) (Ioo 0 n)).Nonempty