fact: Definition fun_of_matrix A (i : 'I_m) (j : 'I_n) := mx_val A (i, j).
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)