proposition: Lemma col_mxKu A1 A2 : usubmx (col_mx A1 A2) = A1.
proof: Proof. by apply/matrixP=> i j; rewrite mxE col_mxEu. 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)