/**
   * Returns a set transformed by applying `f` to each element of `s`.
   */
  def map(s: Set, f: Int => Int): Set = {
    val bound = 10
    def loop(s: Set, f: Int => Int): Set = {
      def iter(a: Int, acc: Set): Set = {
        if (a > bound) acc
        else if(contains(s, a)) {
          if(acc == null) iter(a + 1, singletonSet(f(a)))
          else iter(a + 1, union(acc, singletonSet(f(a))))
        }
        else iter(a + 1, acc)
      }
      iter(-bound, null)
    }
    loop(s, f)
  }