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Syntax Definition wsb 1659
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.)
Hypotheses
Ref Expression
wph wff 𝜑
vx setvar 𝑥
vy setvar 𝑦
Assertion
Ref Expression
wsb wff [𝑦 / 𝑥]𝜑

See definition df-sb 1660 for more information.

Colors of variables: wff set class
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