code: @CanIgnoreReturnValue
    @Override
    public int add(E element, int occurrences) {
      checkIsE(element);
      checkNonnegative(occurrences, "occurrences");
      if (occurrences == 0) {
        return count(element);
      }
      int index = element.ordinal();
      int oldCount = counts[index];
      long newCount = (long) oldCount + occurrences;
      checkArgument(newCount <= Integer.MAX_VALUE, "too many occurrences: %s", newCount);
      counts[index] = (int) newCount;
      if (oldCount == 0) {
        distinctElements++;
      }
      size += occurrences;
      return oldCount;
    }
docs: Modification Operations