fact: Definition delta_mx i0 j0 : 'M[R]_(m, n) := \matrix[delta_mx_key]_(i, j) ((i == i0) && (j == j0))%:R.
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)