def interpolate_history_as_isothermal_intervals(tTpath):
    """Takes the time-temperature path specification and subdivides it for
    calculation in isothermal intervals.
    
    Does it based on model of Ketcham et al., in review.
    It is calibrated to facilitate 0.5% accuracy for end-member F-apatite by
    having a maximum temperature step of 3.5 degrees C when the model
    temperature is within 10 C of the total annealing temperature.  Before this
    cutoff the maximum temperature step required is 8 C.  If the overall model
    time steps are too large, these more distant requirements may not be met.
    """
    total_time = tTpath.totaltime
    rate = tTpath.rate
    abs_rate = abs(rate)
    def_timestep = total_time / 100.0
    return tTpath
