proposition: Lemma Conj_Cinv : forall z : C, z <> zeroC -> Cinv (Conj z) = Cmult (Cinv (Rinj (Rsqr (module z)))) z.
proof: Proof. intros. assert (Conj z <> zeroC); auto with geo. rewrite Cinv_Conj; auto. rewrite Conj_Conj; rewrite module_Conj; auto. Qed.