 Int) = {
    def process(ys: List[Int]): List[Int] =
      if (ys.isEmpty) ys
      else if (ys.head < threshold) ys.head :: process(ys.tail)
      else process(ys.tail)
    process(xs)
  }
  println(filter(List(1, 9, 2, 8, 3, 7, 4), 5))
}
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