Definition df-ndx 12681

Description: Define the structure component index extractor. See Theorem ndxarg 12701 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 12684) another set such as the set of finite ordinals om (df-iom 4627). (Contributed by NM, 4-Sep-2011.)

Assertion

Ref	Expression
df-ndx	|- ndx = ( _I |` NN )

Detailed syntax breakdown of Definition df-ndx

Step	Hyp	Ref	Expression
1		cnx 12675	. 2 class ndx
2		cid 4323	. . 3 class _I
3		cn 8990	. . 3 class NN
4	2, 3	cres 4665	. 2 class ( _I |` NN )
5	1, 4	wceq 1364	1 wff ndx = ( _I |` NN )

This definition is referenced by:  ndxarg  12701