code: def _prepare_document(self):
          
          self._xml = ET.Element("Document")
          self._xml.set("xmlns",
                        "urn:iso:std:iso:20022:tech:xsd:" + self.schema)
          self._xml.set("xmlns:xsi",
                        "http://www.w3.org/2001/XMLSchema-instance")
          ET.register_namespace("",
                                "urn:iso:std:iso:20022:tech:xsd:" + self.schema)
          ET.register_namespace("xsi",
                                "http://www.w3.org/2001/XMLSchema-instance")
          n = ET.Element(self.root_el)
          self._xml.append(n)
docs: Build the main document node and set xml namespaces.