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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 retention_policy_alter(database,
                           name,
                           duration,
                           replication,
                           default=False,
                           user=None,
                           password=None,
                           host=None,
                           port=None):
    '''
    Modify an existing retention policy.

    database
        The database to operate on.

    name
        Name of the policy to modify.

    duration
        How long InfluxDB keeps the data.

    replication
        How many copies of the data are stored in the cluster.

    default
        Whether this policy should be the default or not. Default is False.

    CLI Example:

    .. code-block:: bash

        salt '*' influxdb08.retention_policy_modify metrics default 1d 1
    '''
    client = _client(user=user, password=password, host=host, port=port)
    client.alter_retention_policy(name, database, duration, replication, default)
    return True