proposition: Lemma BZ_of_Z_diff x y: BZ_of_Z (x - y) = (BZ_of_Z x) -z (BZ_of_Z y).
proof: Proof. rewrite BZ_of_Z_sum BZ_of_Z_opp //. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype order) mathcomp(ssrnat ssrint ssralg ssrnum div)