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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 validate_signed_by(message, signer, **config):
 """ Validate that a message was signed by a particular certificate. ``signer`` argument. It will reject a message for any of the regular circumstances, but will also reject it if its not signed by a cert with the argued name. """


    config = copy.deepcopy(config)
    config['routing_nitpicky'] = True
    config['routing_policy'] = {message['topic']: [signer]}
    return validate(message, **config)