proposition: Lemma col_id n j (V : 'cV_n) : col j V = V.
proof: Proof. by apply/colP=> i; rewrite mxE [j]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)