code: @Override public int add(@Nullable E element, int occurrences) {
      if (occurrences == 0) {
        return count(element);
      }
      checkArgument(
          occurrences > 0, "occurrences cannot be negative: %s", occurrences);
      Count frequency = backingMap.get(element);
      int oldCount;
      if (frequency == null) {
        oldCount = 0;
        backingMap.put(element, new Count(occurrences));
      } else {
        oldCount = frequency.get();
        long newCount = (long) oldCount + (long) occurrences;
        checkArgument(newCount <= Integer.MAX_VALUE,
            "too many occurrences: %s", newCount);
        frequency.getAndAdd(occurrences);
      }
      size += occurrences;
      return oldCount;
    }
docs: {@inheritDoc}
  
  @throws IllegalArgumentException if the call would result in more than
  {@link Integer#MAX_VALUE} occurrences of {@code element} in this
  multiset.