Return the serialization format for an element node.

def _element_format(self, occur):
        """Return the serialization format for an element node.""" 
        if occur:
            occ = occur
        else:
            occ = self.occur
        if occ == 1:
            if hasattr(self, "default"):
                self.attr["nma:default"] = self.default
            else:
                self.attr["nma:implicit"] = "true"
        middle = self._chorder() if self.rng_children() else "<empty/>%s"
        fmt = (self.start_tag() + self.serialize_annots().replace("%", "%%") +
               middle + self.end_tag())
        if (occ == 2 or self.parent.name == "choice"
            or self.parent.name == "case" and len(self.parent.children) == 1):
            return fmt
        else:
            return "<optional>" + fmt + "</optional>"