proposition: Lemma invr_eq0 x : (x^-1 == 0) = (x == 0).
proof: Proof. by apply: negb_inj; apply/idP/idP; move/invr_neq0; rewrite ?invrK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)