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(* Specifications: *)
Lemma is_gcd_unic :
forall a b c d : nat, is_gcd a b c -> is_gcd a b d -> c = d.
Lemma is_gcd_ref : forall x : nat, is_gcd x x x.
Lemma is_gcd_sym : forall a b c : nat, is_gcd a b c -> is_gcd b a c.
intros a b c H; inversion_clear H; intros.