proposition: Lemma mxtrace_mxblock (B_ : forall i j, 'M[R]_(p_ i, p_ j)) : \tr (\mxblock_(i, j) B_ i j) = \sum_i \tr (B_ i i).
proof: Proof. rewrite /mxtrace sig_big_dep (reindex _ sig_bij_on)/=. by apply: eq_bigr => i _; 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)