(* fractions form a field *)
Definition RatFieldIdomainMixin := (FieldIdomainMixin field_axiom).Canonical frac_idomainType :=
  Eval hnf in IdomainType type (FieldIdomainMixin field_axiom).Canonical frac_fieldType := FieldType type field_axiom.Notation equivf := (@FracField.equivf _).(* exporting the embeding from R to {fraction R} *)
Local Notation tofrac := (@FracField.tofrac R).

Lemma tofrac_is_additive: additive tofrac.