public void clearIndex(IndexKey key) {
        if (key == IndexKey.SPAN_NODE && annotations instanceof SpanNode2AnnotationContainer) {
            clearIndex();
        } else if (key == IndexKey.ANNOTATION_TYPE && annotations instanceof AnnotationType2AnnotationContainer) {
            clearIndex();
        }
    }
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