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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_waveform_filter_precondition(approximant, length, delta_f):
 """Return the data preconditioning factor for this approximant. """

    if approximant in _filter_preconditions:
        return _filter_preconditions[approximant](length, delta_f)
    else:
        return None