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Theorem exiftruOLD 1658
 Description: Obsolete proof of exiftru 1657 as of 9-Dec-2017. (Contributed by Wolf Lammen, 12-Nov-2017.) (Proof modification is discouraged.) (New usage is discouraged.)
Hypothesis
Ref Expression
exiftruOLD.1 φ
Assertion
Ref Expression
exiftruOLD xφ

Proof of Theorem exiftruOLD
Dummy variable y is distinct from all other variables.
StepHypRef Expression
1 a9ev 1656 . . . 4 x x = y
21a1i 10 . . 3 (x x = yx x = y)
3219.35ri 1602 . 2 x(x = yx = y)
4 exiftruOLD.1 . . . 4 φ
5 id 19 . . . 4 (x = yx = y)
64, 52th 230 . . 3 (φ ↔ (x = yx = y))
76exbii 1582 . 2 (xφx(x = yx = y))
83, 7mpbir 200 1 xφ
 Colors of variables: wff setvar class Syntax hints:   → wi 4  ∀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-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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