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