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Theorem sb9i 1898
Description: Commutation of quantification and substitution variables. (Contributed by NM, 5-Aug-1993.) (Proof rewritten by Jim Kingdon, 23-Mar-2018.)
Assertion
Ref Expression
sb9i  |-  ( A. x [ x  /  y ] ph  ->  A. y [ y  /  x ] ph )

Proof of Theorem sb9i
StepHypRef Expression
1 sb9 1897 . 2  |-  ( A. x [ x  /  y ] ph  <->  A. y [ y  /  x ] ph )
21biimpi 118 1  |-  ( A. x [ x  /  y ] ph  ->  A. y [ y  /  x ] ph )
Colors of variables: wff set class
Syntax hints:    -> wi 4   A.wal 1283   [wsb 1686
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-io 663  ax-5 1377  ax-7 1378  ax-gen 1379  ax-ie1 1423  ax-ie2 1424  ax-8 1436  ax-10 1437  ax-11 1438  ax-i12 1439  ax-bndl 1440  ax-4 1441  ax-17 1460  ax-i9 1464  ax-ial 1468  ax-i5r 1469
This theorem depends on definitions:  df-bi 115  df-nf 1391  df-sb 1687
This theorem is referenced by: (None)
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