Theorem con2b 625

Description: Contraposition. Bidirectional version of con2 604. (Contributed by NM, 5-Aug-1993.)

Assertion

Ref	Expression
con2b	|- ( ( ph -> -. ps ) <-> ( ps -> -. ph ) )

Proof of Theorem con2b

Step	Hyp	Ref	Expression
1		con2 604	. 2 |- ( ( ph -> -. ps ) -> ( ps -> -. ph ) )
2		con2 604	. 2 |- ( ( ps -> -. ph ) -> ( ph -> -. ps ) )
3	1, 2	impbii 124	1 |- ( ( ph -> -. ps ) <-> ( ps -> -. ph ) )

Syntax hints:   -. wn 3   -> wi 4   <-> wb 103

This theorem was proved from axioms:  ax-1 5  ax-2 6  ax-mp 7  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:  mt2bi  641  pm4.15  822  ssconb  3105  disjsn  3454  isprm3  10500