@Override
    void annotate(Annotation a) {
        if (a.getSpanNode() == null) {
            spanNode2Annotation.put(DummySpanNode.INSTANCE, a);
        } else {
            spanNode2Annotation.put(a.getSpanNode(), a);
        }
    }
TODO: Should this be removed?

@author Einar M R Rosenvinge
document/src/main/java/com/yahoo/document/annotation/SpanNode2AnnotationContainer.java