anchor.diff:     */
     void operator []= (E element, int occurences) {
       if (occurences < 0) {
  -      throw new ArgumentError('Negative number of occurences');
  +      throw new ArgumentError('Negative number of occurences: $occurences');
       } else {
         var current = this[element];
         if (occurences > 0) {
  
positive.code:    */
    void add(E element, [int occurences = 1]) {
      if (occurences < 0) {
        throw new ArgumentError('Negative number of occurences');
      } else if (occurences > 0) {
        _container[element] = this[element] + occurences;
        _length += occurences;
  
negative.code:    */
    void remove(Object element, [int occurences = 1]) {
      if (occurences < 0) {
        throw new ArgumentError('Negative number of occurences: $occurences');
      } else if (occurences > 0) {
        var current = this[element];
        if (current <= occurences) {
  