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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 get_aad_content_string(content_type, is_final_frame):
 """Prepares the appropriate Body AAD Value for a message body. :param content_type: Defines the type of content for which to prepare AAD String :type content_type: aws_encryption_sdk.identifiers.ContentType :param bool is_final_frame: Boolean stating whether this is the final frame in a body :returns: Appropriate AAD Content String :rtype: bytes :raises UnknownIdentityError: if unknown content type """

    if content_type == ContentType.NO_FRAMING:
        aad_content_string = ContentAADString.NON_FRAMED_STRING_ID
    elif content_type == ContentType.FRAMED_DATA:
        if is_final_frame:
            aad_content_string = ContentAADString.FINAL_FRAME_STRING_ID
        else:
            aad_content_string = ContentAADString.FRAME_STRING_ID
    else:
        raise UnknownIdentityError("Unhandled content type")
    return aad_content_string