∀ (pat : ByteArray) (stackPos : ℕ) (hst : stackPos < pat.size) (_x : (guess : ℕ) ×' guess < stackPos),
  String.Slice.Pattern.Model.ForwardSliceSearcher.prefixFunctionRecurrence._unary✝ pat stackPos hst _x =
    PSigma.casesOn _x fun guess hg =>
      if pat[guess] = pat[stackPos] then guess + 1
      else
        if h : guess = 0 then 0
        else
          have this := ⋯;
          String.Slice.Pattern.Model.ForwardSliceSearcher.prefixFunctionRecurrence._unary✝¹ pat stackPos hst
            ⟨String.Slice.Pattern.Model.ForwardSliceSearcher.prefixFunction✝ pat (guess - 1) ⋯, ⋯⟩