proposition: Lemma BZpred_sum: (BZpred x +z y) = BZpred (x +z y).
proof: Proof. rewrite /BZpred/BZdiff (BZsumC x) (BZsumC _ (BZopp \1z)). rewrite BZsumA //; apply: (ZSo ZS1). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)