public Collection<IndexKey> getCurrentIndexes() {
        if (annotations instanceof AnnotationType2AnnotationContainer)
            return List.of(IndexKey.ANNOTATION_TYPE);
        if (annotations instanceof SpanNode2AnnotationContainer)
            return List.of(IndexKey.SPAN_NODE);
        return List.of();
    }
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