code: void addTo(Multiset<? super E> target) {
      checkNotNull(target);
      for (int i = backingMap.firstIndex(); i >= 0; i = backingMap.nextIndex(i)) {
        target.add(backingMap.getKey(i), backingMap.getValue(i));
      }
    }
docs: Allocation-free implementation of {@code target.addAll(this)}.