Theorem dfsb7 1991

Description: An alternate definition of proper substitution df-sb 1763. 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 1887, first z for x then y for z. Compare Definition 2.1'' of [Quine] p. 17. Theorem sb7f 1992 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 1887	. . 3 |- ( [ z / x ] ph <-> E. x ( x = z /\ ph ) )
2	1	sbbii 1765	. 2 |- ( [ y / z ] [ z / x ] ph <-> [ y / z ] E. x ( x = z /\ ph ) )
3		ax-17 1526	. . 3 |- ( ph -> A. z ph )
4	3	sbco2vh 1945	. 2 |- ( [ y / z ] [ z / x ] ph <-> [ y / x ] ph )
5		sb5 1887	. 2 |- ( [ y / z ] E. x ( x = z /\ ph ) <-> E. z ( z = y /\ E. x ( x = z /\ ph ) ) )
6	2, 4, 5	3bitr3i 210	1 |- ( [ y / x ] ph <-> E. z ( z = y /\ E. x ( x = z /\ ph ) ) )

Syntax hints:   /\ wa 104   <-> wb 105   E.wex 1492   [wsb 1762

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 106  ax-ia2 107  ax-ia3 108  ax-io 709  ax-5 1447  ax-7 1448  ax-gen 1449  ax-ie1 1493  ax-ie2 1494  ax-8 1504  ax-10 1505  ax-11 1506  ax-i12 1507  ax-4 1510  ax-17 1526  ax-i9 1530  ax-ial 1534  ax-i5r 1535

This theorem depends on definitions:  df-bi 117  df-nf 1461  df-sb 1763

This theorem is referenced by: (None)