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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 _ConvertHeaderToId(header):
 """Convert a Content-ID header value to an id. Presumes the Content-ID header conforms to the format that _ConvertIdToHeader() returns. Args: header: A string indicating the Content-ID header value. Returns: The extracted id value. Raises: BatchError if the header is not in the expected format. """

        if not (header.startswith('<') or header.endswith('>')):
            raise exceptions.BatchError(
                'Invalid value for Content-ID: %s' % header)
        if '+' not in header:
            raise exceptions.BatchError(
                'Invalid value for Content-ID: %s' % header)
        _, request_id = header[1:-1].rsplit('+', 1)

        return urllib_parse.unquote(request_id)