proposition: Lemma unitrX x n : x \is a unit -> x ^+ n \is a unit.
proof: Proof. by move=> Ux; elim: n => [|n IHn]; rewrite ?unitr1 // exprS unitrMl. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)