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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 serialize_non_framed_close(tag, signer=None):
 """Serializes the closing block for a non-framed message body. :param bytes tag: Auth tag value from body encryptor :param signer: Cryptographic signer object (optional) :type signer: aws_encryption_sdk.internal.crypto.Signer :returns: Serialized body close block :rtype: bytes """

    body_close = struct.pack("{auth_len}s".format(auth_len=len(tag)), tag)
    if signer:
        signer.update(body_close)
    return body_close