docs: def to_xml(self):
          '''
          Returns a DOM representation of the deliveries method
          @returns: Element
          '''
          for n, v in { "method": self.method, "status": self.status,
                       "date":self.date}.items():
              if is_empty_or_none(v):
                  raise DeliveryMethodError("'%s' attribute cannot be " \
                                         "empty or None." % n)
  
          doc = Document()
          root = doc.createElement("delivery")
          super(DeliveryMethod, self).to_xml(root)
          self._create_text_node(root, "method", self.method)
          self._create_text_node(root, "status", self.status)
          self._create_text_node(root, "reference", self.ref, True)
          self._create_text_node(root, "date", self.date)
          return root