def METHOD_NAME(registry, ftype="gas", slice_info=None):
    unit_system = registry.ds.unit_system
    # slice_info would be the left, the right, and the factor.
    # For example, with the old Enzo-ZEUS fields, this would be:
    # slice(None, -2, None)
    # slice(1, -1, None)
    # 1.0
    # Otherwise, we default to a centered difference.
    if slice_info is None:
        sl_left = slice(None, -2, None)
        sl_right = slice(2, None, None)
        div_fac = 2.0
    else:
        sl_left, sl_right, div_fac = slice_info
    def _matter_density(field, data):
        return data[ftype, "density"] + data[ftype, "dark_matter_density"]
    registry.add_field(
        (ftype, "matter_density"),
        sampling_type="local",
        function=_matter_density,
        units=unit_system["density"],
    )
    def _matter_mass(field, data):
        return data[ftype, "matter_density"] * data["index", "cell_volume"]
    registry.add_field(
        (ftype, "matter_mass"),
        sampling_type="local",
        function=_matter_mass,
        units=unit_system["mass"],
    )
    # rho_total / rho_cr(z).
    def _overdensity(field, data):
        if (
            not hasattr(data.ds, "cosmological_simulation")
            or not data.ds.cosmological_simulation
        ):
            raise NeedsConfiguration("cosmological_simulation", 1)
        co = data.ds.cosmology
        return data[ftype, "matter_density"] / co.critical_density(
            data.ds.current_redshift
        )
    registry.add_field(
        (ftype, "overdensity"), sampling_type="local", function=_overdensity, units=""
    )
    # rho_baryon / <rho_baryon>
    def _baryon_overdensity(field, data):
        if (
            not hasattr(data.ds, "cosmological_simulation")
            or not data.ds.cosmological_simulation
        ):
            raise NeedsConfiguration("cosmological_simulation", 1)
        omega_baryon = data.get_field_parameter("omega_baryon")
        if omega_baryon is None:
            raise NeedsParameter("omega_baryon")
        co = data.ds.cosmology
        # critical_density(z) ~ omega_lambda + omega_matter * (1 + z)^3
        # mean matter density(z) ~ omega_matter * (1 + z)^3
        return (
            data[ftype, "density"]
            / omega_baryon
            / co.critical_density(0.0)
            / (1.0 + data.ds.current_redshift) ** 3
        )
    registry.add_field(
        (ftype, "baryon_overdensity"),
        sampling_type="local",
        function=_baryon_overdensity,
        units="",
        validators=[ValidateParameter("omega_baryon")],
    )
    # rho_matter / <rho_matter>
    def _matter_overdensity(field, data):
        if (
            not hasattr(data.ds, "cosmological_simulation")
            or not data.ds.cosmological_simulation
        ):
            raise NeedsConfiguration("cosmological_simulation", 1)
        co = data.ds.cosmology
        # critical_density(z) ~ omega_lambda + omega_matter * (1 + z)^3
        # mean density(z) ~ omega_matter * (1 + z)^3
        return (
            data[ftype, "matter_density"]
            / data.ds.omega_matter
            / co.critical_density(0.0)
            / (1.0 + data.ds.current_redshift) ** 3
        )
    registry.add_field(
        (ftype, "matter_overdensity"),
        sampling_type="local",
        function=_matter_overdensity,
        units="",
    )
    # r / r_vir
    def _virial_radius_fraction(field, data):
        virial_radius = data.get_field_parameter("virial_radius")
        if virial_radius == 0.0:
            ret = 0.0
        else:
            ret = data[("index", "radius")] / virial_radius
        return ret
    registry.add_field(
        ("index", "virial_radius_fraction"),
        sampling_type="local",
        function=_virial_radius_fraction,
        validators=[ValidateParameter("virial_radius")],
        units="",
    )
    # Weak lensing convergence.
    # Eqn 4 of Metzler, White, & Loken (2001, ApJ, 547, 560).
    # This needs to be checked for accuracy.
    def _weak_lensing_convergence(field, data):
        if (
            not hasattr(data.ds, "cosmological_simulation")
            or not data.ds.cosmological_simulation
        ):
            raise NeedsConfiguration("cosmological_simulation", 1)
        co = data.ds.cosmology
        pc = data.ds.units.physical_constants
        observer_redshift = data.get_field_parameter("observer_redshift")
        source_redshift = data.get_field_parameter("source_redshift")
        # observer to lens
        dl = co.angular_diameter_distance(observer_redshift, data.ds.current_redshift)
        # observer to source
        ds = co.angular_diameter_distance(observer_redshift, source_redshift)
        # lens to source
        dls = co.angular_diameter_distance(data.ds.current_redshift, source_redshift)
        # removed the factor of 1 / a to account for the fact that we are projecting
        # with a proper distance.
        return (
            1.5
            * (co.hubble_constant / pc.clight) ** 2
            * (dl * dls / ds)
            * data[ftype, "matter_overdensity"]
        ).in_units("1/cm")
    registry.add_field(
        (ftype, "weak_lensing_convergence"),
        sampling_type="local",
        function=_weak_lensing_convergence,
        units=unit_system["length"] ** -1,
        validators=[
            ValidateParameter("observer_redshift"),
            ValidateParameter("source_redshift"),
        ],
    )