proposition: Lemma ZmuS_prod a b: inc a BZm -> inc b BZm -> inc (a *z b) BZp.
proof: Proof. case /setU1_P; last by move => -> _ ; rewrite BZprod_0l; apply: ZpS0. move => am;case /setU1_P; last by move -> ; rewrite BZprod_0r; apply: ZpS0. move => bm; exact :( BZps_sBZp (ZmsuS_prod am bm)). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)