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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 check_stf_agent(adbprefix=None, kill=False):
    '''return True if agent is alive.'''
    if adbprefix is None: 
        adbprefix = ['adb']
    command = adbprefix + ['shell', 'ps']
    out = subprocess.check_output(command).strip()
    out = out.splitlines()
    if len(out) > 1:
        first, out = out[0], out[1:]
        idx = first.split().index('PID')
        pid = None
        for line in out:
            if 'stf.agent' in line:
                pid = line.split()[idx]
                print 'stf.agent is running, pid is', pid
                break
        if pid is not None:
            if kill:
                print 'killing', pid
                command = adbprefix + ['shell', 'kill', '-9', pid]
                subprocess.call(command)
                return False
            return True
    return False