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