Qed.
Inductive sum_t (A B : Type) : Type :=
| inl_t : A -> sum_t A B
| inr_t : B -> sum_t A B.
Hint Resolve inl_t inr_t: zfc.

Theorem Paire_CIN :
forall E E' A : Ens, CIN A (Paire E E') -> sum_t (EQC A E) (EQC A E').