public SpanTree annotate(Annotation a) {
        if (a.getSpanNode() == null) {
            annotateInternal(DummySpanNode.INSTANCE, a);
        } else {
            annotateInternal(a.getSpanNode(), a);
        }
        return this;
    }
Adds an Annotation to the internal list of annotations for this SpanTree.&nbsp;Use this when
adding an Annotation that uses an AnnotationReference, and does not annotate a SpanNode.

@param a the Annotation to add
@return this, for chaining
@see com.yahoo.document.annotation.Annotation
@see com.yahoo.document.annotation.AnnotationReference
@see com.yahoo.document.annotation.AnnotationReferenceDataType
document/src/main/java/com/yahoo/document/annotation/SpanTree.java