Theorem negcon2i

Description: Negative contraposition law. (Contributed by NM, 25-Aug-1999)

	Ref	Expression
Hypotheses	negidi.1	|- A e. CC
	pncan3i.2	|- B e. CC
Assertion	negcon2i	|- ( A = -u B <-> B = -u A )

Proof

Step	Hyp	Ref	Expression
1		negidi.1	|- A e. CC
2		pncan3i.2	|- B e. CC
3		negcon2	|- ( ( A e. CC /\ B e. CC ) -> ( A = -u B <-> B = -u A ) )
4	1 2 3	mp2an	|- ( A = -u B <-> B = -u A )