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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 mobility(sdat, tstart=None, tend=None):
 """Plates mobility. Compute the ratio vsurf / vrms. Args: sdat (:class:`~stagpy.stagyydata.StagyyData`):
 a StagyyData instance. tstart (float):
 time at which the computation should start. Use the beginning of the time series data if set to None. tend (float):
 time at which the computation should end. Use the end of the time series data if set to None. Returns: tuple of :class:`numpy.array`: mobility and time arrays. """

    tseries = sdat.tseries_between(tstart, tend)
    steps = sdat.steps[tseries.index[0]:tseries.index[-1]]
    time = []
    mob = []
    for step in steps.filter(rprof=True):
        time.append(step.timeinfo['t'])
        mob.append(step.rprof.iloc[-1].loc['vrms'] / step.timeinfo['vrms'])
    return np.array(mob), np.array(time)