Definition C_Ring_mixin :=
  Ring.Mixin _ _ _ Cmult_assoc Cmult_1_r Cmult_1_l Cmult_plus_distr_r Cmult_plus_distr_l.Canonical C_Ring :=
  Ring.Pack C (Ring.Class _ _ C_Ring_mixin) C.

Lemma Cmod_m1 : Cmod (Copp 1) = 1.