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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_norm(approximant, psd, length, delta_f, f_lower):
 """ Return the normalization vector for the approximant """

    if approximant in _filter_norms:
        return _filter_norms[approximant](psd, length, delta_f, f_lower)
    else:
        return None