proposition: Lemma mxOver_const c S : c \in S -> const_mx c \is a mxOver S.
proof: Proof. by move=> cS; apply/mxOverP => i j; rewrite !mxE. 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)