rw [countelements]
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
⊢ (filter (fun x => x ∈ {n} - B) (Ioo 0 n)).card = countelements B (n - 1)
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
⊢ (filter (fun x => x ∈ {n} - B) (Ioo 0 n)).card = (filter (fun x => x ∈ B) (Icc 1 (n - 1))).card