proposition: Lemma BZsum_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 => xN yN; rewrite (BZsum_pp (BZ_of_natp_i xN) (BZ_of_natp_i yN)). by rewrite !BZ_of_nat_val. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)