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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _define_format(self, occur):
 """Return the serialization format for a define node."""
 
        if hasattr(self, "default"):
            self.attr["nma:default"] = self.default
        middle = self._chorder() if self.rng_children() else "<empty/>%s"
        return (self.start_tag() + self.serialize_annots().replace("%", "%%")
                + middle + self.end_tag())