code: def toTag(self, output):
          ''' This methods adds all data of this canteen as canteen xml tag
          to the given xml Document.
          :meth:`toXMLFeed` uses this method to create the XML Feed. So there is
          normally no need to call it directly.
          :param output: XML Document to which the data should be added
          :type output: xml.dom.DOMImplementation.createDocument
          '''
          canteen = output.createElement('canteen')
          if self._name is not None:
              canteen.appendChild(self._buildStringTag('name', self._name, output))
          if self._address is not None:
              canteen.appendChild(self._buildStringTag('address', self._address, output))
          if self._city is not None:
              canteen.appendChild(self._buildStringTag('city', self._city, output))
          if self._phone is not None:
              canteen.appendChild(self._buildStringTag('phone', self._phone, output))
          if self._email is not None:
              canteen.appendChild(self._buildStringTag('email', self._email, output))
          if self._location is not None:
              canteen.appendChild(self._buildLocationTag(self._location, output))
          if self._availability is not None:
              canteen.appendChild(self._buildStringTag('availability', self._availability, output))
          for feed in sorted(self.feeds, key=lambda v: v.priority):
              canteen.appendChild(feed.toTag(output))
          for date in sorted(self._days.keys()):
              day = output.createElement('day')
              day.setAttribute('date', str(date))
              if self._days[date] is False:
                  closed = output.createElement('closed')
                  day.appendChild(closed)
                  canteen.appendChild(day)
                  continue
              for categoryname in self._days[date]:
                  day.appendChild(self._buildCategoryTag(
                      categoryname, self._days[date][categoryname], output))
              canteen.appendChild(day)
          return canteen