for-comprehension produces Seq instead of List

  type Occurrences = List[(Char, Int)]

    def expand(acc: List[Occurrences], occurrence: (Char, Int)): List[Occurrences] = occurrence match {
      case (c, int) => (for (i <- (0 to int) ; ao <- acc) yield ((c, i) :: ao)).toList
    }

// I have to use toList yet result is empty anyway

expand(List[Occurrences](), ('a' ,3))         //> List()