proposition: Lemma rowK u_ i0 : row i0 (\matrix_i u_ i) = u_ i0.
proof: Proof. by apply/rowP=> i'; rewrite !mxE. 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)