Theorem nbn 647

Description: The negation of a wff is equivalent to the wff's equivalence to falsehood. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Wolf Lammen, 3-Oct-2013.)

Hypothesis

Ref	Expression
nbn.1	|- -. ph

Assertion

Ref	Expression
nbn	|- ( -. ps <-> ( ps <-> ph ) )

Proof of Theorem nbn

Step	Hyp	Ref	Expression
1		nbn.1	. . 3 |- -. ph
2		bibif 646	. . 3 |- ( -. ph -> ( ( ps <-> ph ) <-> -. ps ) )
3	1, 2	ax-mp 7	. 2 |- ( ( ps <-> ph ) <-> -. ps )
4	3	bicomi 130	1 |- ( -. ps <-> ( ps <-> ph ) )

Syntax hints:   -. wn 3   <-> wb 103

This theorem was proved from axioms:  ax-1 5  ax-2 6  ax-mp 7  ax-ia1 104  ax-ia2 105  ax-ia3 106  ax-in1 576  ax-in2 577

This theorem depends on definitions:  df-bi 115

This theorem is referenced by:  nbn3  648  nbfal  1295  n0rf  3260  eq0  3266  disj  3292  dm0rn0  4570  reldm0  4571