public final SpanTree annotateFast(SpanNode node, Annotation annotation) {
        annotateInternal(node, annotation);
        return this;
    }
Adds an Annotation to the internal list of annotations for this SpanTree. Use this when
adding an Annotation that shall annotate a SpanNode. Upon return, Annotation.getSpanNode()
returns the given node. This one is unchecked and assumes that the SpanNode is valid and has
already been attached to the Annotation.

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