proposition: Lemma mxtrace_block n1 n2 (Aul : 'M_n1) Aur Adl (Adr : 'M_n2) : \tr (block_mx Aul Aur Adl Adr) = \tr Aul + \tr Adr.
proof: Proof. rewrite /(\tr _) big_split_ord /=. by congr (_ + _); under eq_bigr do rewrite (block_mxEul, block_mxEdr). 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)