def METHOD_NAME(example_zpk_fs_tuple):
    z, p, k, fs = example_zpk_fs_tuple
    b, a = signal.zpk2tf(z, p, k)
    # this should be converted to ZPK form
    dform, dfilt = filter_design.convert_to_digital((b, a), fs)
    assert dform == 'zpk'
    assert dfilt == filter_design.bilinear_zpk(z, p, k, fs)