proposition: Lemma map2_castmx m' n' c : map2_mx (castmx c A) (castmx c B) = castmx c (map2_mx A B) :> '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)