proposition: Lemma countable_setX2 a b: countable_set a -> countable_set b -> countable_set (a \times b).
proof: Proof. move => /countableP ca /countableP cb; apply/countableP. rewrite - (cprod2_pr1 a b)- cprod2cl - cprod2cr. exact: (cprod_inf4 ca cb CIS_aleph0). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)