public SpanNode annotate(Annotation annotation) {
        getNonNullSpanTree().annotate(this, annotation);
        return this;
    }
Convenience method for adding an annotation to this span, same as
<code>getSpanTree().{@link SpanTree#annotate(SpanNode,Annotation) spanTree.annotate(this,annotation)}</code>

@param annotation the annotation to add
@return this for chaining
@throws NullPointerException if this span is not attached to a tree
document/src/main/java/com/yahoo/document/annotation/SpanNode.java