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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 _case_format(self, occur):
 """Return the serialization format for a case node."""
 
        if self.occur == 1:
            self.attr["nma:implicit"] = "true"
        ccnt = len(self.rng_children())
        if ccnt == 0: return "<empty/>%s"
        if ccnt == 1 or not self.interleave:
            return self.start_tag("group") + "%s" + self.end_tag("group")
        return (self.start_tag("interleave") + "%s" +
                self.end_tag("interleave"))