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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 _serialize_call(self, format_, call):
 """Return serialized version of the Call using the record's FORMAT'"""

        if isinstance(call, record.UnparsedCall):
            return call.unparsed_data
        else:
            result = [
                format_value(self.header.get_format_field_info(key), call.data.get(key), "FORMAT")
                for key in format_
            ]
            return ":".join(result)