Empty list as identity eleent in collector of lists

  def listWithoutRepetitions(s: Int, used: List[Int], available: List[Int], acc: List[List[Int]]): List[List[Int]] = {
    if (s == 0) used :: acc else
       if (s < 0 || available.isEmpty) acc
        else available match {
          case d :: ds => {
            val left = listWithoutRepetitions(s, used, ds, acc)
            listWithoutRepetitions(s-d, d :: used, ds, left)
          }
        }
  }                                               //> listWithoutRepetitions: (s: Int, used: List[Int], available: List[Int], acc
                                                  //| : List[List[Int]])List[List[Int]]

  countWithoutRepetitions(10, List(1, 2, 3, 5))   //> res1: Int = 1
  listWithoutRepetitions(10, List(), List(1, 2, 3, 5), Nil : List[List[Int]])

The problem is that I am getting empty list in the output always, in addition to the items collected. Assignment https://class.coursera.org/progfun-004/assignment/view?assignment_id=17 misteriously says "Note that the anagram of the empty sentence is the empty sentence itself." I do not know what does it mean.