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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_footer(signer):
 """Uses the signer object which has been used to sign the message to generate the signature, then serializes that signature. :param signer: Cryptographic signer object :type signer: aws_encryption_sdk.internal.crypto.Signer :returns: Serialized footer :rtype: bytes """

    footer = b""
    if signer is not None:
        signature = signer.finalize()
        footer = struct.pack(">H{sig_len}s".format(sig_len=len(signature)), len(signature), signature)
    return footer