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

Lemma Build_Ctype_comp2 :
forall x x' y y' : R,
Equal x x' -> Equal y y' -> Ceq (Build_Ctype x y) (Build_Ctype x' y').