fact: Definition diag_mx n (d : 'rV[V]_n) := \matrix[diag_mx_key]_(i, j) (d 0 i *+ (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)