Theorem dfor2

Description: Logical 'or' expressed in terms of implication only. Theorem *5.25 of WhiteheadRussell p. 124. (Contributed by NM, 12-Aug-2004) (Proof shortened by Wolf Lammen, 20-Oct-2012)

	Ref	Expression
Assertion	dfor2	|- ( ( ph \/ ps ) <-> ( ( ph -> ps ) -> ps ) )

Proof

Step	Hyp	Ref	Expression
1		pm2.62	|- ( ( ph \/ ps ) -> ( ( ph -> ps ) -> ps ) )
2		pm2.68	|- ( ( ( ph -> ps ) -> ps ) -> ( ph \/ ps ) )
3	1 2	impbii	|- ( ( ph \/ ps ) <-> ( ( ph -> ps ) -> ps ) )