proposition: Lemma col_rsubmx A i : col i (rsubmx A) = col (rshift n1 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)