}

    private ValueHolder addHolder() {
      ValueHolder valueHolder = new ValueHolder();
      holderTail = holderTail.next = valueHolder;
      return valueHolder;
    }

    @CanIgnoreReturnValue
    private ToStringHelper addHolder(@CheckForNull Object value) {
      ValueHolder valueHolder = addHolder();
      valueHolder.value = value;
      return this;
    }