proposition: Lemma mxsub_comp m1 m2 m3 n1 n2 n3 (f : 'I_m2 -> 'I_m1) (f' : 'I_m3 -> 'I_m2) (g : 'I_n2 -> 'I_n1) (g' : 'I_n3 -> 'I_n2) (A : 'M_(m1, n1)) : mxsub (f \o f') (g \o g') A = mxsub f' g' (mxsub f g A).
proof: Proof. by apply/matrixP => i j; 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)