private void clearIndex() {
        AnnotationContainer tmpAnnotations = new ListAnnotationContainer();
        tmpAnnotations.annotateAll(annotations.annotations());
        annotations = tmpAnnotations;
    }
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