proposition: Lemma invr_eq1 x : (x^-1 == 1) = (x == 1).
proof: Proof. by rewrite (inv_eq invrK) invr1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)