code: public TreeGraphNode followArcToNode(Class<? extends GrammaticalRelationAnnotation> arcLabel) {
      Set<TreeGraphNode> valueSet = followArcToSet(arcLabel);
      if (valueSet == null) {
        return null;
      }
      return valueSet.iterator().next();
    }
docs: Returns a single <code>TreeGraphNode</code> to which there
  exists an arc bearing the specified label from this node, or
  <code>null</code> if no such node exists.  If more than one
  such node exists, this method will return an arbitrary node
  from among them; if this is a possibility, you might want to
  use {@link TreeGraphNode#followArcToSet
  <code>followArcToSet</code>} instead.
  
  @param arcLabel a <code>Object</code> containing the label of
  the arc(s) to be followed
  @return a <code>TreeGraphNode</code> to which there exists an
  arc bearing the specified label from this node