public SpanNode annotate(AnnotationType type) {
        getNonNullSpanTree().annotate(this, type);
        return this;
    }
Convenience method for adding an annotation with no value to this span, same as
<code>getSpanTree().{@link SpanTree#annotate(SpanNode,AnnotationType) spanTree.annotate(this,type)}</code>

@param type the type of 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