proposition: Lemma commrN x y : comm x y -> comm x (- y).
proof: Proof. by move=> com_xy; rewrite /comm mulrN com_xy mulNr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)