code: def _case_format(self, occur):
           
          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"))
docs: Return the serialization format for a case node.