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Take any function F from ordinals to ordinals. Then, we define:

  • ψ0F(0)[0] = 0
  • ψβ+1F(0) = min{x|"x is regular"∧∀y<min{z|z=ψzF(0)},w<β(ψwF(y),cof(ψwF(y))<x)}
  • ψβ+1F(0)[n] = n
  • cof(β) ≥ ω: ψβF(0)[n] = ψβ[n]F(0)
  • α+1 > 0: ψβF(α+1)[0] = ψβF(α)+1
  • α+1 > 0: ψβF(α+1)[n+1] = F(ψβF(α+1)[n])
  • ω ≤ cof(α) < ψβ+1F(0): ψβF(α)[n] = ψβF(α[n])
  • cof(α) = ψβ+1F(0): ψβF(α)[0] = 0
  • cof(α) = ψβ+1F(0): ψβF(α)[n+1] = ψβF(α[ψβF(α)[n]])
  • cof(α) > ψβ+1F(0): ψβF(α) = ψβFβ+1F(α))

The cofinalities of some ordinals are not taken into account and unspecified, but these are either (1) supposed to already be known by the reader or (2) able to be deduced from the ruleset. (NOTE: There are no extended FSes, like ω2[ω+1] or Ω[Ω2] (although something like Ω2[Ω] is fine, as the FS of an ordinal has the same length as its cofinality))

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