∀x:ℕ+. ∀y:ℕ. (gcd(x;y mod x) = gcd(x;y) ∈ ℤ)
{ (Auto THEN CaseNat 0 `y') }
1. x : ℕ+
2. y : ℕ
3. y = 0 ∈ ℤ
⊢ gcd(x;0 mod x) = gcd(x;0) ∈ ℤ
3. ¬(y = 0 ∈ ℤ)
⊢ gcd(x;y mod x) = gcd(x;y) ∈ ℤ