proposition: Lemma mul_copid_mx_pid m n r : r <= m -> copid_mx r *m pid_mx r = 0 :> 'M_(m, n).
proof: Proof. by move=> le_r_m; rewrite mulmxBl mul1mx pid_mx_id ?subrr. 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)