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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 add_authorized_key(public_key, application_name, user=None):
 """Add given key to the authorized_key file. :param public_key: Public key. :type public_key: str :param application_name: Name of application eg nova-compute-something :type application_name: str :param user: The user that the ssh asserts are for. :type user: str """

    with open(authorized_keys(application_name, user), 'a') as keys:
        keys.write("{}\n".format(public_key))