(* fractions form a ring with explicit unit *)
Canonical frac_unitRingType := Eval hnf in UnitRingType type RatFieldUnitMixin.
Canonical frac_comUnitRingType := [comUnitRingType of type].

Lemma tofracB : {morph tofrac: x y / x - y}. Proof. exact: rmorphB. Qed.