proposition: Lemma unitrPr x : reflect (exists y, x * y = 1) (x \in unit).
proof: Proof. by apply: (iffP (unitrP x)) => [[y []] | [y]]; exists y; rewrite // mulrC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)