@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) {

  static long checkNonnegative(long value, String name) {
    if (value < 0) {
      throw new IllegalArgumentException(name + " cannot be negative but was: " + value);
    }
    return value;
  }

  static void checkPositive(int value, String name) {
    if (value <= 0) {
      throw new IllegalArgumentException(name + " must be positive but was: " + value);
    }
  }

  /**