@Override
    public void addItem(Item item) {
        if (item instanceof WordItem) {
            addWordItem((WordItem) item);
        } else {
            throw new IllegalArgumentException("Can not add " + item + " to a segment phrase");
        }
    }
Adds subitem. The word will have its index name set to the index name
of this phrase. If the item is a word, it will simply be added,
if the item is a phrase, each of the words of the phrase will be added.

@throws IllegalArgumentException if the given item is not a WordItem or PhraseItem
container-search/src/main/java/com/yahoo/prelude/query/PhraseSegmentItem.java