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Syntax Definition wsb 1642
Description: Extend wff definition to include proper substitution (read "the wff that results when y is properly substituted for x in wff φ"). (Contributed by NM, 24-Jan-2006.)
Ref Expression
wph wff φ
vx setvar x
vy setvar y
Ref Expression
wsb wff [y / x]φ

See definition df-sb 1643 for more information.

Colors of variables: wff set class
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