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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_interface(zone, interface, permanent=True):
    '''
    Bind an interface to a zone

    .. versionadded:: 2016.3.0

    CLI Example:

    .. code-block:: bash

        salt '*' firewalld.add_interface zone eth0
    '''
    if interface in get_interfaces(zone, permanent):
        log.info('Interface is already bound to zone.')

    cmd = '--zone={0} --add-interface={1}'.format(zone, interface)

    if permanent:
        cmd += ' --permanent'

    return __firewall_cmd(cmd)