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(**********     Doubletons  ************)
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Definition Doubleton (X Y : Ens) := union (Singleton X) (Singleton Y).Definition doubleton_th1_Out : IN X D :=
  union_th1 (Singleton X) (Singleton Y) X iXsX.

(* Specifications: *)
Theorem Doubleton_th1 : forall X Y : Ens, IN X (Doubleton X Y).