PPaste!

  def wordOccurrences(w: Word): Occurrences = {
    val chars = w.toLowerCase.toList
    val map = chars.groupBy((element: Char) => element) map {case(c, e) =>(c, e.length)}
    map.toList.sorted
  }

  /** Converts a sentence into its character occurrence list. */
  def sentenceOccurrences(s: Sentence): Occurrences = wordOccurrences(s.foldLeft("")(_ ++ _))

  lazy val dictionaryByOccurrences: Map[Occurrences, List[Word]] =
    dictionary groupBy wordOccurrences