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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 list_services(zone=None, permanent=True):
    '''
    List services added for zone as a space separated list.
    If zone is omitted, default zone will be used.

    CLI Example:

    .. code-block:: bash

        salt '*' firewalld.list_services

    List a specific zone

    .. code-block:: bash

        salt '*' firewalld.list_services my_zone
    '''
    if zone:
        cmd = '--zone={0} --list-services'.format(zone)
    else:
        cmd = '--list-services'

    if permanent:
        cmd += ' --permanent'

    return __firewall_cmd(cmd).split()