proposition: Lemma row_mxsub m1 m2 n1 n2 (f : 'I_m2 -> 'I_m1) (g : 'I_n2 -> 'I_n1) (A : 'M_(m1, n1)) i : row i (mxsub f g A) = row (f i) (colsub g A).
proof: Proof. by rewrite !rowEsub -!mxsub_comp. 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)