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_add :
forall a b c : nat, congruent a b -> congruent (a + c) (b + c).