code: public  Node selectSingleNode(Node contextNode, String str)
            throws TransformerException
    {
      return selectSingleNode(contextNode, str, contextNode);
    }
docs: Use an XPath string to select a single node. XPath namespace
  prefixes are resolved from the context node, which may not
  be what you want (see the next method).
  
  @param contextNode The node to start searching from.
  @param str A valid XPath string.
  @return The first node found that matches the XPath, or null.
  
  @throws TransformerException