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Theorem fveqvfvv 46469
Description: If a function's value at an argument is the universal class (which can never be the case because of fvex 6915), the function's value at this argument is any set (especially the empty set). In short "If a function's value is a proper class, it is a set", which sounds strange/contradictory, but which is a consequence of that a contradiction implies anything (see pm2.21i 119). (Contributed by Alexander van der Vekens, 26-May-2017.)
Assertion
Ref Expression
fveqvfvv ((𝐹𝐴) = V → (𝐹𝐴) = 𝐵)

Proof of Theorem fveqvfvv
StepHypRef Expression
1 fvex 6915 . . . 4 (𝐹𝐴) ∈ V
2 eleq1a 2824 . . . 4 ((𝐹𝐴) ∈ V → (V = (𝐹𝐴) → V ∈ V))
31, 2ax-mp 5 . . 3 (V = (𝐹𝐴) → V ∈ V)
4 vprc 5319 . . . 4 ¬ V ∈ V
54pm2.21i 119 . . 3 (V ∈ V → (𝐹𝐴) = 𝐵)
63, 5syl 17 . 2 (V = (𝐹𝐴) → (𝐹𝐴) = 𝐵)
76eqcoms 2736 1 ((𝐹𝐴) = V → (𝐹𝐴) = 𝐵)
Colors of variables: wff setvar class
Syntax hints:  wi 4   = wceq 1533  wcel 2098  Vcvv 3473  cfv 6553
This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1789  ax-4 1803  ax-5 1905  ax-6 1963  ax-7 2003  ax-8 2100  ax-9 2108  ax-ext 2699  ax-sep 5303  ax-nul 5310
This theorem depends on definitions:  df-bi 206  df-an 395  df-or 846  df-tru 1536  df-fal 1546  df-ex 1774  df-sb 2060  df-clab 2706  df-cleq 2720  df-clel 2806  df-ne 2938  df-v 3475  df-dif 3952  df-un 3954  df-in 3956  df-ss 3966  df-nul 4327  df-sn 4633  df-pr 4635  df-uni 4913  df-iota 6505  df-fv 6561
This theorem is referenced by:  afvpcfv0  46573
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