code: private void addOnlyChildElements() {
  		for (Node node : nodes) {
  			Index index = helper.getIndexInParent(node, false);
  			if (index.size==1)
  				result.add(node);
  		}
  	}
docs: Add {@code :only-child} elements.
  
  @see <a href="http://www.w3.org/TR/css3-selectors/#only-child-pseudo"><code>:only-child</code> pseudo-class</a>