proposition: Lemma exprVn x n : x^-1 ^+ n = x ^- n.
proof: Proof. elim: n => [|n IHn]; first by rewrite !expr0 ?invr1. case Ux: (x \is a unit); first by rewrite exprSr exprS IHn -invrM // unitrX. by rewrite !invr_out ?unitrX_pos ?Ux. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)