How to use the pynbody.snapshot.gadgethdf.SubFindHDFSnap.derived_quantity function in pynbody

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def doppler_redshift(sim) :
    """Doppler Redshift from LoS Velocity 'losvel' using SR """

    return np.sqrt( (1. + sim['losvel'].in_units('c')) / (1. - sim['losvel'].in_units('c'))  ) - 1.

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def OVI(sim) :
    """Fraction of Oxygen OVI"""

    import pynbody.analysis.ionfrac

    return pynbody.analysis.ionfrac.calculate(sim.g,ion='ovi')

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def sixh(self) :
    minsi = np.amin(self['Si'][np.where(self['Si'] > 0)])
    self['Si'][np.where(self['Si'] == 0)]=minsi
    return np.log10(self['Si']/self['Si']) - np.log10(XSOLSi/XSOLH)

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def nexh(self) :
    minne = np.amin(self['Ne'][np.where(self['Ne'] > 0)])
    self['Ne'][np.where(self['Ne'] == 0)]=minne
    return np.log10(self['Ne']/self['Ne']) - np.log10(XSOLNe/XSOLH)

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def SIV(sim) :
    """Fraction of Silicon SiIV"""

    import pynbody.analysis.ionfrac

    return pynbody.analysis.ionfrac.calculate(sim.g,ion='siiv')

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def em(sim) :
    """Emission Measure (n_e^2) per particle to be integrated along LoS"""

    return sim.g["rho_ne"]*sim.g["rho_ne"]

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def HI(sim) :
    """Fraction of Neutral Hydrogen HI use limited CLOUDY table"""

    import pynbody.analysis.hifrac

    return pynbody.analysis.hifrac.calculate(sim.g,ion='hi')

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def mgxh(self) :
    minmg = np.amin(self['Mg'][np.where(self['Mg'] > 0)])
    self['Mg'][np.where(self['Mg'] == 0)]=minmg
    return np.log10(self['Mg']/self['Mg']) - np.log10(XSOLMg/XSOLH)

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def ne(sim) :
    """Number of electrons per proton mass, ignoring the contribution from He!"""
    ne = sim.g["HII"]  #+ sim["HeII"] + 2*sim["HeIII"]
    ne.units = units.m_p**-1

    return ne

@GadgetHDFSnap.derived_quantity
@SubFindHDFSnap.derived_quantity
def p(sim) :
    """Calculate the pressure for gas particles, including polytropic equation of state gas"""

    critpres = 2300. * units.K * units.m_p / units.cm**3 ## m_p K cm^-3
    critdens = 0.1 * units.m_p / units.cm**3 ## m_p cm^-3
    gammaeff = 4./3.

    oneos = sim.g['OnEquationOfState'] == 1.

    p = sim.g['rho'].in_units('m_p cm**-3') * sim.g['temp'].in_units('K')
    p[oneos] = critpres * (sim.g['rho'][oneos].in_units('m_p cm**-3')/critdens)**gammaeff

    return p