public List<Span> getSubSpans() {
        return subSpans == null ?
                Collections.<Span>emptyList() :
                Collections.unmodifiableList(subSpans);
    }
Returns all spans that are directly childrens of this span. If the span is a leaf, the empty
list will be returned. The list is unmodifable.
container-search/src/main/java/com/yahoo/text/interpretation/Span.java