Section powerplus2.
Variable A B X Y : Ens.
Hypothesis x : IN X A.
Hypothesis y : IN Y B.
Let U := union A B.
Let step1 : SUB A U.
Proof union_pr1 A B.
Let step2 : SUB B U.
Proof union_pr2 A B.
Let step3 : IN X U.
Proof step1 X x.
Let step4 : IN Y U.
Proof step2 Y y.

Lemma powerplus2_Out : IN (PAIR X Y) (PowerTotal U).