Remark cPairInjHelp :
 forall a b c d : nat, cPair a b = cPair c d -> a + b = c + d.

(* Specifications: *)
Lemma cPairInj1 : forall a b c d : nat, cPair a b = cPair c d -> a = c.
Lemma cPairInj2 : forall a b c d : nat, cPair a b = cPair c d -> b = d.