Occurrences

def wordOccurrences(w: Word): Occurrences = {
    val occMap = w.toLowerCase groupBy (c => c) mapValues( _.size )
    occMap.toList
  }

  /** Converts a sentence into its character occurrence list. */
  def sentenceOccurrences(s: Sentence): Occurrences = {
    val wordOccs = for ( word <- s ) yield wordOccurrences( word )
    wordOccs.toList.flatten
  }