proposition: Lemma col_lsubmx A i : col i (lsubmx A) = col (lshift n2 i) A.
proof: Proof. by apply/colP=> j; rewrite !mxE; congr (A _ _); apply/val_inj. 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)