fact: Variable f : {linear 'M[R]_(m1, n1) -> 'M[R]_(m2, n2)}.
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)