Return the serialization format for a _list_ node.

def _list_format(self, occur):
        """Return the serialization format for a _list_ node.""" 
        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_ + ">")