proposition: Fact mulC_unitP x y : y * x = 1 /\ x * y = 1 -> unit x.
proof: Proof. by case=> yx _; apply: unitPl yx. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)