code: protected Node xpathFindImpl(String xpath, NamespaceContext nsContext)
          throws XPathExpressionException
      {
      	Node foundNode = (Node) this.xpathQuery(xpath, XPathConstants.NODE, nsContext);
      	if (foundNode == null || foundNode.getNodeType() != Node.ELEMENT_NODE) {
      		throw new XPathExpressionException("XPath expression \""
      			+ xpath + "\" does not resolve to an Element in context "
      			+ this.xmlNode + ": " + foundNode);
      	}
      	return foundNode;
      }
docs: Find the first element in the builder's DOM matching the given
  XPath expression, where the expression may include namespaces if
  a {@link NamespaceContext} is provided.
  
  @param xpath
  An XPath expression that *must* resolve to an existing Element within
  the document object model.
  @param nsContext
  a mapping of prefixes to namespace URIs that allows the XPath expression
  to use namespaces.
  
  @return
  the first Element that matches the XPath expression.
  
  @throws XPathExpressionException
  If the XPath is invalid, or if does not resolve to at least one
  {@link Node#ELEMENT_NODE}.