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Metamath Proof Explorer

Theorem pm4.57

Description: Theorem *4.57 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

Ref Expression
Assertion pm4.57 
|- ( -. ( -. ph /\ -. ps ) <-> ( ph \/ ps ) )

Proof

Step Hyp Ref Expression
1 oran 
 |-  ( ( ph \/ ps ) <-> -. ( -. ph /\ -. ps ) )
2 1 bicomi 
 |-  ( -. ( -. ph /\ -. ps ) <-> ( ph \/ ps ) )