proposition: Lemma unitrV x : (x^-1 \in unit) = (x \in unit).
proof: Proof. by rewrite !unitrE invrK commrV. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)