code: def assemble_content_aad(message_id, aad_content_string, seq_num, length):
      
      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)
docs: 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