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Definition df-ndx 11999
Description: Define the structure component index extractor. See theorem ndxarg 12019 to understand its purpose. The restriction to  NN ensures that  ndx is a set. The restriction to some set is necessary since  _I is a proper class. In principle, we could have chosen  CC or (if we revise all structure component definitions such as df-base 12002) another set such as the set of finite ordinals 
om (df-iom 4512). (Contributed by NM, 4-Sep-2011.)
Assertion
Ref Expression
df-ndx  |-  ndx  =  (  _I  |`  NN )

Detailed syntax breakdown of Definition df-ndx
StepHypRef Expression
1 cnx 11993 . 2  class  ndx
2 cid 4217 . . 3  class  _I
3 cn 8743 . . 3  class  NN
42, 3cres 4548 . 2  class  (  _I  |`  NN )
51, 4wceq 1332 1  wff  ndx  =  (  _I  |`  NN )
Colors of variables: wff set class
This definition is referenced by:  ndxarg  12019
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