public void addPositiveItem(Item item) {
        if (getPositiveItem() instanceof TrueItem) {
            setPositiveItem(item);
        } else if (getPositiveItem() instanceof AndItem) {
            ((AndItem) getPositiveItem()).addItem(item);
        } else {
            AndItem positives = new AndItem();

            positives.addItem(getPositiveItem());
            positives.addItem(item);
            setPositiveItem(positives);
        }
    }
Convenience method for adding a positive item.
If a positive item is already present
the positive item becomes an AndItem with the items added
container-search/src/main/java/com/yahoo/prelude/query/NotItem.java