code: 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);
    }
docs: Returns the label of a single arc between this node and <code>destNode</code>,
  or <code>null</code> if no such arc exists.  If more than one
  such arc exists, this method will return an arbitrary arc label
  from among them; if this is a possibility, you might want to
  use {@link TreeGraphNode#arcLabelsToNode
  <code>arcLabelsToNode</code>} instead.
  
  @param destNode the destination node
  @return the <code>Object</code> which
  labels one arc between this node and <code>destNode</code>