gistfile1.scala

val a = ('a', 2)                                //> a  : (Char, Int) = (a,2)
  val b = ('b', 2)                                //> b  : (Char, Int) = (b,2)
  val c = ('c', 3)                                //> c  : (Char, Int) = (c,3)
  val d = ('d', 1)                                //> d  : (Char, Int) = (d,1)
  val list = List(a, b, c, d)                     //> list  : List[(Char, Int)] = List((a,2), (b,2), (c,3), (d,1))
  val sub = List(('a', 1), ('b', 4), ('c', 3))    //> sub  : List[(Char, Int)] = List((a,1), (b,4), (c,3))
  
  def combo(sub: Occurrences): List[Occurrences] = {
  	val acc: List[Occurrences] = List(List())
  	sub match {
  	case Nil => acc
  	case (char, freq) :: Nil =>
  	acc ::: (for {
  		f <- 1 to freq
  	} yield List((char, f))).toList
  	case occ :: rest => for {
  		rst <- combo(rest)
  		f <- 1 to occ._2
  	} yield (occ._1, f) :: rst
  }
}