Inductive gcd_spec : nat -> nat -> nat -> Prop :=
  | gcd_spec_ex0 : forall a : nat, gcd_spec a 0 a
  | gcd_spec_ex1 : forall b : nat, gcd_spec 0 b b
  | gcd_spec_ex2 :
      forall a b c : nat, a < b -> gcd_spec a (b - a) c -> gcd_spec a b c
  | gcd_spec_ex3 :
      forall a b c : nat, b <= a -> gcd_spec (a - b) b c -> gcd_spec a b c.Inductive gcd_spec : nat -> nat -> nat -> Prop :=
  | gcd_spec_ex0 : forall a : nat, gcd_spec a 0 a
  | gcd_spec_ex1 : forall b : nat, gcd_spec 0 b b
  | gcd_spec_ex2 :
      forall a b c : nat, a < b -> gcd_spec a (b - a) c -> gcd_spec a b c
  | gcd_spec_ex3 :
      forall a b c : nat, b <= a -> gcd_spec (a - b) b c -> gcd_spec a b c.Inductive gcd_spec : nat -> nat -> nat -> Prop :=
  | gcd_spec_ex0 : forall a : nat, gcd_spec a 0 a
  | gcd_spec_ex1 : forall b : nat, gcd_spec 0 b b
  | gcd_spec_ex2 :
      forall a b c : nat, a < b -> gcd_spec a (b - a) c -> gcd_spec a b c
  | gcd_spec_ex3 :
      forall a b c : nat, b <= a -> gcd_spec (a - b) b c -> gcd_spec a b c.Inductive gcd_spec : nat -> nat -> nat -> Prop :=
  | gcd_spec_ex0 : forall a : nat, gcd_spec a 0 a
  | gcd_spec_ex1 : forall b : nat, gcd_spec 0 b b
  | gcd_spec_ex2 :
      forall a b c : nat, a < b -> gcd_spec a (b - a) c -> gcd_spec a b c
  | gcd_spec_ex3 :
      forall a b c : nat, b <= a -> gcd_spec (a - b) b c -> gcd_spec a b c.Inductive gcd_spec : nat -> nat -> nat -> Prop :=
  | gcd_spec_ex0 : forall a : nat, gcd_spec a 0 a
  | gcd_spec_ex1 : forall b : nat, gcd_spec 0 b b
  | gcd_spec_ex2 :
      forall a b c : nat, a < b -> gcd_spec a (b - a) c -> gcd_spec a b c
  | gcd_spec_ex3 :
      forall a b c : nat, b <= a -> gcd_spec (a - b) b c -> gcd_spec a b c.

Theorem gcd_inv_Ol : forall a b : nat, gcd_spec 0 a b -> a = b.