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Definition df-iota 5358
Description: Define Russell's definition description binder, which can be read as "the unique  x such that  ph," where  ph ordinarily contains  x as a free variable. Our definition is meaningful only when there is exactly one  x such that  ph is true (see iotaval 5369); otherwise, it evaluates to the empty set (see iotanul 5373). Russell used the inverted iota symbol 
iota to represent the binder.

Sometimes proofs need to expand an iota-based definition. That is, given "X = the x for which ... x ... x ..." holds, the proof needs to get to "... X ... X ...". A general strategy to do this is to use riotacl2 6499 (or iotacl 5381 for unbounded iota), as demonstrated in the proof of supub 7397. This can be easier than applying riotasbc 6501 or a version that applies an explicit substitution, because substituting an iota into its own property always has a bound variable clash which must be first renamed or else guarded with NF.

(Contributed by Andrew Salmon, 30-Jun-2011.)

Assertion
Ref Expression
df-iota  |-  ( iota
x ph )  =  U. { y  |  {
x  |  ph }  =  { y } }
Distinct variable groups:    x, y    ph, y
Allowed substitution hint:    ph( x)

Detailed syntax breakdown of Definition df-iota
StepHypRef Expression
1 wph . . 3  wff  ph
2 vx . . 3  set  x
31, 2cio 5356 . 2  class  ( iota
x ph )
41, 2cab 2373 . . . . 5  class  { x  |  ph }
5 vy . . . . . . 7  set  y
65cv 1648 . . . . . 6  class  y
76csn 3757 . . . . 5  class  { y }
84, 7wceq 1649 . . . 4  wff  { x  |  ph }  =  {
y }
98, 5cab 2373 . . 3  class  { y  |  { x  | 
ph }  =  {
y } }
109cuni 3957 . 2  class  U. {
y  |  { x  |  ph }  =  {
y } }
113, 10wceq 1649 1  wff  ( iota
x ph )  =  U. { y  |  {
x  |  ph }  =  { y } }
Colors of variables: wff set class
This definition is referenced by:  dfiota2  5359  iotaeq  5366  iotabi  5367  dffv4  5665  dfiota3  25486
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