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Syntax Definition wsb 1630
Description: Extend wff definition to include proper substitution (read "the wff that results when  y is properly substituted for  x in wff  ph"). (Contributed by NM, 24-Jan-2006.)
Hypotheses
Ref Expression
wph  wff  ph
vx  set  x
vy  set  y
Assertion
Ref Expression
wsb  wff  [ y  /  x ] ph

See definition df-sb 1631 for more information.

Colors of variables: wff set class
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