Inductive sum_t (A B : Type) : Type :=
  | inl_t : A -> sum_t A B
  | inr_t : B -> sum_t A B.Inductive sum_t (A B : Type) : Type :=
  | inl_t : A -> sum_t A B
  | inr_t : B -> sum_t A B.Inductive sum_t (A B : Type) : Type :=
  | inl_t : A -> sum_t A B
  | inr_t : B -> sum_t A B.

Theorem CIN_Sing_EQ : forall E E' : Ens, CIN E (Sing E') -> EQC E E'.