private void addWordItem(WordItem word) {
        word.setIndexName(this.getIndexName());
        super.addItem(word);
    }
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