proposition: Lemma copid_mx_id n r : r <= n -> copid_mx r *m copid_mx r = copid_mx r :> 'M_n.
proof: Proof. by move=> le_r_n; rewrite mulmxBl mul1mx mul_pid_mx_copid // oppr0 addr0. 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)