Theorem pm4.81 357

Description: Theorem *4.81 of [WhiteheadRussell] p. 122. (Contributed by NM, 3-Jan-2005.)

Assertion

Ref	Expression
pm4.81	|- ( ( -. ph -> ph ) <-> ph )

Proof of Theorem pm4.81

Step	Hyp	Ref	Expression
1		pm2.18 104	. 2 |- ( ( -. ph -> ph ) -> ph )
2		pm2.24 103	. 2 |- ( ph -> ( -. ph -> ph ) )
3	1, 2	impbii 182	1 |- ( ( -. ph -> ph ) <-> ph )

Syntax hints:   -. wn 5   -> wi 6   <-> wb 178

This theorem was proved from axioms:  ax-1 7  ax-2 8  ax-3 9  ax-mp 10

This theorem depends on definitions:  df-bi 179