fact: Definition nz_row m n (A : 'M_(m, n)) := oapp (fun i => row i A) 0 [pick i | row i A != 0].
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)