@Override
    public Optional<Item> extractSingleChild() {
        Optional<Item> extracted = super.extractSingleChild();
        extracted.ifPresent(e -> e.setWeight(this.getWeight()));
        return extracted;
    }
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