code: @CanIgnoreReturnValue
    @Override
    public final int add(@NullableDecl E element, int occurrences) {
      if (occurrences == 0) {
        return count(element);
      }
      checkArgument(occurrences > 0, "occurrences cannot be negative: %s", occurrences);
      int entryIndex = backingMap.indexOf(element);
      if (entryIndex == -1) {
        backingMap.put(element, occurrences);
        size += occurrences;
        return 0;
      }
      int oldCount = backingMap.getValue(entryIndex);
      long newCount = (long) oldCount + (long) occurrences;
      checkArgument(newCount <= Integer.MAX_VALUE, "too many occurrences: %s", newCount);
      backingMap.setValue(entryIndex, (int) newCount);
      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.