return a readable XML form of the data.

def workbook_data(self):
        """return a readable XML form of the data."""
        document = XML(
            fn=os.path.splitext(self.fn)[0]+'.xml', 
            root=Element.workbook())
        shared_strings = [
            str(t.text) for t in 
            self.xml('xl/sharedStrings.xml')
                .root.xpath(".//xl:t", namespaces=self.NS)]
        for key in self.sheets.keys():
            worksheet = self.sheets[key].transform(XT, shared_strings=shared_strings)
            document.root.append(worksheet.root)
        return document