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

Lemma tofrac_is_multiplicative: multiplicative tofrac.
Canonical tofrac_rmorphism := AddRMorphism tofrac_is_multiplicative.