Proof union_th1_OutA.
(************************************************)
(**********     Elementary Counting  ************)
(************************************************)
Definition has_leq_0_elements := Its_empty.
(*** recall that this is equal to [A: Ens] (X: Ens) (not (IN X A)) ***)

Theorem zero_has_leq_0_elements : has_leq_0_elements Zero.
Proof EmptySet_th1.