docs: public Class<? extends GrammaticalRelationAnnotation> arcLabelToNode(TreeGraphNode destNode) {
      Set<Class<? extends GrammaticalRelationAnnotation>> arcLabels = arcLabelsToNode(destNode);
      if (arcLabels == null) {
        return null;
      }
      return (new ArrayList<Class<? extends GrammaticalRelationAnnotation>>(arcLabels)).get(0);
    }