code: public AnnotationNode annotation(String annotation) {
          this.attr(ANNOTATION, annotation);
          String[] kc = AnnotationParser.extractKeyAndContent(annotation);
          this.attr(ANNOTATION_KEY, kc[0]);
          this.attr(ANNOTATION_CONTENT, kc[1]);
          return this;
      }
docs: Set the annotation of this node.
  @param annotation raw annotation
  @return this, for chaining