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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 _get_shells():
    '''
    Return the valid shells on this system
    '''
    start = time.time()
    if 'sh.last_shells' in __context__:
        if start - __context__['sh.last_shells'] > 5:
            __context__['sh.last_shells'] = start
        else:
            __context__['sh.shells'] = __salt__['cmd.shells']()
    else:
        __context__['sh.last_shells'] = start
        __context__['sh.shells'] = __salt__['cmd.shells']()
    return __context__['sh.shells']