code: def postcode(self):
          
          postcode = ''
          pattern = self.random_element(self.postcode_formats)
          for placeholder in pattern:
              postcode += self.random_element(self._postcode_sets[placeholder])
          return postcode
docs: See
          http://web.archive.org/web/20090930140939/http://www.govtalk.gov.uk/gdsc/html/noframes/PostCode-2-1-Release.htm