Fact unitGI_out : {in [predC unitGI], invGI =1 id}.
Definition GI_comUnitRingMixin :=
ComUnitRingMixin mulGIr unitGIP unitGI_out.
Canonical GI_unitRingType := UnitRingType GI GI_comUnitRingMixin.
Canonical GI_comUnitRingType := [comUnitRingType of GI].

(**  ** Question 4: Show that gauss integers are stable by conjugation.  *)
Lemma conjGIE x : (x^* \is a gaussInteger) = (x \is a gaussInteger).