proposition: Fact mxOver_opp_subproof : oppr_closed (@mxOver m n _ oppS).
proof: Proof. by move=> A /mxOverP SA; apply/mxOverP=> i j; rewrite mxE rpredN. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup perm order) mathcomp(div prime binomial ssralg countalg finalg zmodp)