@Override
    public int compareTo(SpanTree spanTree) {
        int comp = name.compareTo(spanTree.name);
        if (comp != 0) {
            comp = root.compareTo(spanTree.root);
        }
        return comp;
    }
Returns the StringFieldValue that this SpanTree belongs to.

@return the StringFieldValue that this SpanTree belongs to, if any, otherwise null.
document/src/main/java/com/yahoo/document/annotation/SpanTree.java