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Definition df-iota 6060
Description: Define Russell's definition description binder, which can be read as "the unique 𝑥 such that 𝜑," where 𝜑 ordinarily contains 𝑥 as a free variable. Our definition is meaningful only when there is exactly one 𝑥 such that 𝜑 is true (see iotaval 6071); otherwise, it evaluates to the empty set (see iotanul 6075). Russell used the inverted iota symbol 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 6844 (or iotacl 6083 for unbounded iota), as demonstrated in the proof of supub 8600. This can be easier than applying riotasbc 6846 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 (℩𝑥𝜑) = {𝑦 ∣ {𝑥𝜑} = {𝑦}}
Distinct variable groups:   𝑥,𝑦   𝜑,𝑦
Allowed substitution hint:   𝜑(𝑥)

Detailed syntax breakdown of Definition df-iota
StepHypRef Expression
1 wph . . 3 wff 𝜑
2 vx . . 3 setvar 𝑥
31, 2cio 6058 . 2 class (℩𝑥𝜑)
41, 2cab 2792 . . . . 5 class {𝑥𝜑}
5 vy . . . . . . 7 setvar 𝑦
65cv 1636 . . . . . 6 class 𝑦
76csn 4370 . . . . 5 class {𝑦}
84, 7wceq 1637 . . . 4 wff {𝑥𝜑} = {𝑦}
98, 5cab 2792 . . 3 class {𝑦 ∣ {𝑥𝜑} = {𝑦}}
109cuni 4630 . 2 class {𝑦 ∣ {𝑥𝜑} = {𝑦}}
113, 10wceq 1637 1 wff (℩𝑥𝜑) = {𝑦 ∣ {𝑥𝜑} = {𝑦}}
Colors of variables: wff setvar class
This definition is referenced by:  dfiota2  6061  iotaeq  6068  iotabi  6069  dffv4  6401  dfiota3  32346  reuabaiotaiota  41665
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