proposition: Lemma unitrM x y : (x * y \in unit) = (x \in unit) && (y \in unit).
proof: Proof. exact/unitrM_comm/mulrC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)