unfold Ceq in |- *.
intros x x' H'; elim H'; intros H'0 H'1; try exact H'0; clear H'.
Qed.
Hint Resolve real_comp: algebra.

(* Specifications: *)
Lemma imag_comp : forall x x' : Cset, Equal x x' -> Equal (imag x) (imag x').
simpl in |- *.