code: def _list_format(self, occur):
           
          if self.keys:
              self.attr["nma:key"] = " ".join(self.keys)
              keys = ''.join([self.keymap[k].serialize(occur=2)
                              for k in self.keys])
          else:
              keys = ""
          if self.maxEl:
              self.attr["nma:max-elements"] = self.maxEl
          if int(self.minEl) == 0:
              ord_ = "zeroOrMore"
          else:
              ord_ = "oneOrMore"
              if int(self.minEl) > 1:
                  self.attr["nma:min-elements"] = self.minEl
          middle = self._chorder() if self.rng_children() else "<empty/>%s"
          return ("<" + ord_ + ">" + self.start_tag("element") +
                  (self.serialize_annots() + keys).replace("%", "%%")  +
                  middle + self.end_tag("element") + "</" + ord_ + ">")
docs: Return the serialization format for a _list_ node.