@CanIgnoreReturnValue
  @Override
  public int add(E element, int occurrences) {
    checkNotNull(element);
    if (occurrences == 0) {
      return count(element);
    }
    CollectPreconditions.checkPositive(occurrences, "occurrences");

    while (true) {
      AtomicInteger existingCounter = safeGet(countMap, element);
      if (existingCounter == null) {