Syntax Definition wsb 1648

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 1649 for more information.