proposition: Fact unitr_sdivr_closed : @sdivr_closed R unit.
proof: Proof. by split=> [|x y Ux Uy]; rewrite ?unitrN1 // unitrMl ?unitrV. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)