proposition: Lemma map_castmx m' n' c : (castmx c A)^f = castmx c A^f :> 'M_(m', n').
proof: Proof. by apply/matrixP=> i j; rewrite !(castmxE, 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)