fact: Definition vec_mx (u : 'rV[R]_(m * n)) := \matrix[vec_mx_key]_(i, j) u 0 (mxvec_index 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)