human
Here's a good function header given the following:
I wrote the below code

    assert_is_spectral_unit(rest)
    ckms = _si.c.to_value(u'km/s')
    def to_vel_freq(x):
        restfreq = rest.to_value(si.Hz, equivalencies=spectral())
        return ((((restfreq ** 2) - (x ** 2)) / ((restfreq ** 2) + (x ** 2))) * ckms)
    def from_vel_freq(x):
        restfreq = rest.to_value(si.Hz, equivalencies=spectral())
        voverc = (x / ckms)
        return (restfreq * (((1 - voverc) / (1 + voverc)) ** 0.5))
    def to_vel_wav(x):
        restwav = rest.to_value(si.AA, spectral())
        return ((((x ** 2) - (restwav ** 2)) / ((restwav ** 2) + (x ** 2))) * ckms)
    def from_vel_wav(x):
        restwav = rest.to_value(si.AA, spectral())
        voverc = (x / ckms)
        return (restwav * (((1 + voverc) / (1 - voverc)) ** 0.5))
    def to_vel_en(x):
        resten = rest.to_value(si.eV, spectral())
        return ((((resten ** 2) - (x ** 2)) / ((resten ** 2) + (x ** 2))) * ckms)
    def from_vel_en(x):
        resten = rest.to_value(si.eV, spectral())
        voverc = (x / ckms)
        return (resten * (((1 - voverc) / (1 + voverc)) ** 0.5))
    return [(si.Hz, (si.km / si.s), to_vel_freq, from_vel_freq), (si.AA, (si.km / si.s), to_vel_wav, from_vel_wav), (si.eV, (si.km / si.s), to_vel_en, from_vel_en)]

What's a good function header?
assistant
#Here's a good function header.

def doppler_relativistic(rest):