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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 _set_persistent_module(mod):
    '''
    Add a module to loader.conf to make it persistent.
    '''
    if not mod or mod in mod_list(True) or mod not in \
            available():
        return set()
    __salt__['file.append'](_LOADER_CONF, _LOAD_MODULE.format(mod))
    return set([mod])