Theorem pm5.19 375

Description: Theorem *5.19 of [WhiteheadRussell] p. 124. (Contributed by NM, 3-Jan-2005.)

Assertion

Ref	Expression
pm5.19	|- -. ( ph <-> -. ph )

Proof of Theorem pm5.19

Step	Hyp	Ref	Expression
1		biid 251	. 2 |- ( ph <-> ph )
2		pm5.18 371	. 2 |- ( ( ph <-> ph ) <-> -. ( ph <-> -. ph ) )
3	1, 2	mpbi 220	1 |- -. ( ph <-> -. ph )

Syntax hints:   -. wn 3   <-> wb 196

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8

This theorem depends on definitions:  df-bi 197

This theorem is referenced by:  xorexmid  1480  ru  3434  pwfseqlem1  9480  bisym1  32418  bj-ru0  32932  rusbcALT  38640