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Theorem 19.9vOLD 1697
Description: Obsolete version of 19.9v 1664 as of 4-Dec-2017. (Contributed by NM, 28-May-1995.) (Revised by NM, 1-Aug-2017.) (New usage is discouraged.) (Proof modification is discouraged.)
Assertion
Ref Expression
19.9vOLD (xφφ)
Distinct variable group:   φ,x

Proof of Theorem 19.9vOLD
StepHypRef Expression
1 df-ex 1542 . 2 (xφ ↔ ¬ x ¬ φ)
2 19.3v 1665 . . 3 (x ¬ φ ↔ ¬ φ)
32con2bii 322 . 2 (φ ↔ ¬ x ¬ φ)
41, 3bitr4i 243 1 (xφφ)
Colors of variables: wff setvar class
Syntax hints:  ¬ wn 3  wb 176  wal 1540  wex 1541
This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1546  ax-5 1557  ax-17 1616  ax-9 1654
This theorem depends on definitions:  df-bi 177  df-an 360  df-ex 1542
This theorem is referenced by: (None)
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