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Metamath Proof Explorer

Theorem pm4.55

Description: Theorem *4.55 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

Ref Expression
Assertion pm4.55 
|- ( -. ( -. ph /\ ps ) <-> ( ph \/ -. ps ) )

Proof

Step Hyp Ref Expression
1 pm4.54 
 |-  ( ( -. ph /\ ps ) <-> -. ( ph \/ -. ps ) )
2 1 con2bii 
 |-  ( ( ph \/ -. ps ) <-> -. ( -. ph /\ ps ) )
3 2 bicomi 
 |-  ( -. ( -. ph /\ ps ) <-> ( ph \/ -. ps ) )