proposition: Lemma row_sum_delta n (u : 'rV_n) : u = \sum_(j < n) u 0 j *: delta_mx 0 j.
proof: Proof. by rewrite {1}[u]matrix_sum_delta big_ord1. Qed.
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)