fact: Definition Zgcd a b := match a, b with | Z0, _ => b | _, Z0 => a | Zpos a, Zneg b => Zpos (gcd a b) | Zneg a, Zpos b => Zpos (gcd a b) | Zpos a, Zpos b => Zpos (gcd a b) | Zneg a, Zneg b => Zpos (gcd a b) end.
imports: ZArith Lia ZCmisc Zwf ZArith Znumtheory