apply le_trans with (cPair (cPairPi1 a) (cPairPi2 a)).
apply cPairLe1.
rewrite cPairProjections.
apply le_n.
Qed.

(* Specifications: *)
Lemma cPairLe2 : forall a b : nat, b <= cPair a b.
Lemma cPairLe2A : forall a : nat, cPairPi2 a <= a.
intros.