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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 mark_time(times, msg=None):
 """ Time measurement utility. Measures times of execution between subsequent calls using time.clock(). The time is printed if the msg argument is not None. Examples -------- elapsed 0.1 elapsed again 0.05 [0.10000000000000001, 0.050000000000000003] """

    tt = time.clock()
    times.append(tt)
    if (msg is not None) and (len(times) > 1):
        print msg, times[-1] - times[-2]