Section CD.
Variable n : nat.
(**********************************************************************)
Inductive congruent : nat -> nat -> Prop :=
congruentDef :
forall a b u v : nat, a + u * n = b + v * n -> congruent a b.

Lemma cong_mult_O : forall a b : nat, congruent a 0 -> congruent (a * b) 0.