code: def lowpass(cutoff):
    
    R = thub(exp(cutoff - pi), 2)
    G = (R + 1) / 2
    return G * (1 + z ** -1) / (1 + R * z ** -1)
docs: This strategy uses an exponential approximation for cut-off frequency
    calculation, found by matching the single pole and single zero Laplace
    highpass filter and mirroring the resulting filter to get a lowpass.