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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 assemble_content_aad(message_id, aad_content_string, seq_num, length):
 """Assembles the Body AAD string for a message body structure. :param message_id: Message ID :type message_id: str :param aad_content_string: ContentAADString object for frame type :type aad_content_string: aws_encryption_sdk.identifiers.ContentAADString :param seq_num: Sequence number of frame :type seq_num: int :param length: Content Length :type length: int :returns: Properly formatted AAD bytes for message body structure. :rtype: bytes :raises SerializationError: if aad_content_string is not known """

    if not isinstance(aad_content_string, aws_encryption_sdk.identifiers.ContentAADString):
        raise SerializationError("Unknown aad_content_string")
    fmt = ">16s{}sIQ".format(len(aad_content_string.value))
    return struct.pack(fmt, message_id, aad_content_string.value, seq_num, length)