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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_queues(backend='sqlite'):
    '''
    Return a list of Salt Queues on the backend

    CLI Example:

    .. code-block:: bash

        salt-run queue.list_queues
        salt-run queue.list_queues backend=sqlite
    '''
    queue_funcs = salt.loader.queues(__opts__)
    cmd = '{0}.list_queues'.format(backend)
    if cmd not in queue_funcs:
        raise SaltInvocationError('Function "{0}" is not available'.format(cmd))
    ret = queue_funcs[cmd]()
    return ret