Theorem cvjust 2172

Description: Every set is a class. Proposition 4.9 of [TakeutiZaring] p. 13. This theorem shows that a setvar variable can be expressed as a class abstraction. This provides a motivation for the class syntax construction cv 1352, which allows us to substitute a setvar variable for a class variable. See also cab 2163 and df-clab 2164. Note that this is not a rigorous justification, because cv 1352 is used as part of the proof of this theorem, but a careful argument can be made outside of the formalism of Metamath, for example as is done in Chapter 4 of Takeuti and Zaring. See also the discussion under the definition of class in [Jech] p. 4 showing that "Every set can be considered to be a class." (Contributed by NM, 7-Nov-2006.)

Assertion

Ref	Expression
cvjust	|- x = { y | y e. x }

Distinct variable group:   x, y

Proof of Theorem cvjust

Dummy variable z is distinct from all other variables.

Step	Hyp	Ref	Expression
1		dfcleq 2171	. 2 |- ( x = { y | y e. x } <-> A. z ( z e. x <-> z e. { y | y e. x } ) )
2		df-clab 2164	. . 3 |- ( z e. { y | y e. x } <-> [ z / y ] y e. x )
3		elsb1 2155	. . 3 |- ( [ z / y ] y e. x <-> z e. x )
4	2, 3	bitr2i 185	. 2 |- ( z e. x <-> z e. { y | y e. x } )
5	1, 4	mpgbir 1453	1 |- x = { y | y e. x }

Syntax hints:   <-> wb 105   = wceq 1353   [wsb 1762   e. wcel 2148   {cab 2163

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-bndl 1509  ax-4 1510  ax-17 1526  ax-i9 1530  ax-ial 1534  ax-i5r 1535  ax-13 2150  ax-ext 2159

This theorem depends on definitions:  df-bi 117  df-nf 1461  df-sb 1763  df-clab 2164  df-cleq 2170

This theorem is referenced by: (None)