proposition: Lemma divKr x : x \is a unit -> {in unit, involutive (fun y => x / y)}.
proof: Proof. by move=> Ux y Uy; rewrite /= invrM ?unitrV // invrK mulrC divrK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)