proposition: Lemma unitrE x : (x \is a unit) = (x / x == 1).
proof: Proof. apply/idP/eqP=> [Ux | xx1]; first exact: divrr. by apply/unitrP; exists x^-1; rewrite -commrV. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)