∀ {s : String.Slice} (needle : String.Slice) (table : Vector ℕ needle.utf8ByteSize)
  (ht : table = String.Slice.Pattern.ForwardSliceSearcher.buildTable needle) (stackPos needlePos : String.Pos.Raw)
  (hn : needlePos < needle.rawEndPos),
  String.Slice.Pattern.ForwardSliceSearcher.toOption✝
      (String.Slice.Pattern.ForwardSliceSearcher.proper needle table ht stackPos needlePos hn) =
    some (s.utf8ByteSize - stackPos.byteIdx, needlePos.byteIdx)