public Item setPositiveItem(Item item) {
        Objects.requireNonNull(item, () -> "Positive item of " + this);
        if (getItemCount() == 0) {
            addItem(item);
            return null;
        } else {
            return setItem(0, item);
        }
    }
Sets the positive item (the first item)

@return the old positive item, or TrueItem if there was none
container-search/src/main/java/com/yahoo/prelude/query/NotItem.java