Theorem negcon1i

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

	Ref	Expression
Hypotheses	negidi.1	|- A e. CC
	pncan3i.2	|- B e. CC
Assertion	negcon1i	|- ( -u A = B <-> -u B = A )

Proof

Step	Hyp	Ref	Expression
1		negidi.1	|- A e. CC
2		pncan3i.2	|- B e. CC
3		negcon1	|- ( ( A e. CC /\ B e. CC ) -> ( -u A = B <-> -u B = A ) )
4	1 2 3	mp2an	|- ( -u A = B <-> -u B = A )