proposition: Lemma row_usubmx A i : row i (usubmx A) = row (lshift m2 i) A.
proof: Proof. by apply/rowP=> 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)