proposition: Lemma BZprod_cN x y: natp x -> natp y -> BZ_of_nat x *z BZ_of_nat y = BZ_of_nat (x *c y).
proof: Proof. move => pa pb. rewrite (BZprod_pp (BZ_of_natp_i pa) (BZ_of_natp_i pb)). by rewrite !BZ_of_nat_val. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)