contradiction
case inl
A B : Set ℕ
hA : 0 ∈ A
hB : 0 ∈ B
hc : 0 ≤ countelements A 0 + countelements B 0
h : 0 ∉ A + B
hnA : 0 ∉ A
hnB : 0 ∉ B
hca : countelements A (0 - 1) = countelements A 0
hcb : countelements B (0 - 1) = countelements B 0
⊢ False
no goals