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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 _prepare_headers( self, headers: Optional[LooseHeaders]) -> 'CIMultiDict[str]': """ Add default headers and transform it to CIMultiDict """

        # Convert headers to MultiDict
        result = CIMultiDict(self._default_headers)
        if headers:
            if not isinstance(headers, (MultiDictProxy, MultiDict)):
                headers = CIMultiDict(headers)
            added_names = set()  # type: Set[str]
            for key, value in headers.items():
                if key in added_names:
                    result.add(key, value)
                else:
                    result[key] = value
                    added_names.add(key)
        return result