proposition: Lemma mxsubcr m1 m2 n n2 f g (A : 'M_(m1, n)) : mxsub f g A = colsub g (rowsub f A) :> 'M_(m2, n2).
proof: Proof. exact: 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)