Theorem dfsb7 1964

Description: An alternate definition of proper substitution df-sb 1736. By introducing a dummy variable z in the definiens, we are able to eliminate any distinct variable restrictions among the variables x, y, and ph of the definiendum. No distinct variable conflicts arise because z effectively insulates x from y. To achieve this, we use a chain of two substitutions in the form of sb5 1859, first z for x then y for z. Compare Definition 2.1'' of [Quine] p. 17. Theorem sb7f 1965 provides a version where ph and z don't have to be distinct. (Contributed by NM, 28-Jan-2004.)

Assertion

Ref	Expression
dfsb7	|- ( [ y / x ] ph <-> E. z ( z = y /\ E. x ( x = z /\ ph ) ) )

Distinct variable groups:   x, z   y, z   ph, z

Allowed substitution hints:   ph( x, y)

Proof of Theorem dfsb7

Step	Hyp	Ref	Expression
1		sb5 1859	. . 3 |- ( [ z / x ] ph <-> E. x ( x = z /\ ph ) )
2	1	sbbii 1738	. 2 |- ( [ y / z ] [ z / x ] ph <-> [ y / z ] E. x ( x = z /\ ph ) )
3		ax-17 1506	. . 3 |- ( ph -> A. z ph )
4	3	sbco2vh 1916	. 2 |- ( [ y / z ] [ z / x ] ph <-> [ y / x ] ph )
5		sb5 1859	. 2 |- ( [ y / z ] E. x ( x = z /\ ph ) <-> E. z ( z = y /\ E. x ( x = z /\ ph ) ) )
6	2, 4, 5	3bitr3i 209	1 |- ( [ y / x ] ph <-> E. z ( z = y /\ E. x ( x = z /\ ph ) ) )

Syntax hints:   /\ wa 103   <-> wb 104   E.wex 1468   [wsb 1735

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 105  ax-ia2 106  ax-ia3 107  ax-io 698  ax-5 1423  ax-7 1424  ax-gen 1425  ax-ie1 1469  ax-ie2 1470  ax-8 1482  ax-10 1483  ax-11 1484  ax-i12 1485  ax-4 1487  ax-17 1506  ax-i9 1510  ax-ial 1514  ax-i5r 1515

This theorem depends on definitions:  df-bi 116  df-nf 1437  df-sb 1736

This theorem is referenced by: (None)