@Nullable
    public static XPathExpression getXPathExpression(XsltElement xsltElement, String attributeName) {
        final XmlAttribute attribute = xsltElement.getTag().getAttribute(attributeName, null);
        if (attribute != null) {
            final PsiFile[] files = XsltSupport.getFiles(attribute);
            if (files.length > 0) {
                assert files.length == 1 : "Unexpected number of XPathFiles in @" + attributeName + ": " + Arrays.toString(files);
                return PsiTreeUtil.getChildOfType(files[0], XPathExpression.class);
            }
        }
        return null;
    }
@Nullable
    public static XPathExpression getXPathExpression(XsltElement xsltElement, String attributeName)