proposition: Lemma BZprod_opp_opp x y: intp x -> intp y -> (BZopp x) *z (BZopp y) = x *z y.
proof: Proof. by move => pa pb; rewrite (BZprod_opp_comm (ZSo pa) pb) BZopp_K. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)