proposition: Lemma BZprodBr x y z: intp x -> intp y -> intp z -> x *z (y -z z) = (x *z y) -z (x *z z).
proof: Proof. move => xz yz zz; rewrite /BZdiff (BZprodDr xz yz (ZSo zz)). by rewrite BZopp_prod_r. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)