How to register a system in pint?

Question

I want to define a system programmitcally. For example, once I define

        "real": Units(
            mass="g/mol",
            distance="angstrom",
            time="fs",
            energy="kcal/mol",
            charge="e",
            velocity="angstrom/fs",
            force="kcal/(mol*angstrom)",
            torque="kcal/mol",
            temperature="K",
            pressure="atm",
            dynamic_viscosity="poise",
            electric_field="V/angstrom",
            density="g/cm^3"
        )

I can express any value with unit in this way

boltzmann: J/K -> kcal/mol/K

How should I do?

---

Another related question is, if I define fundational units, such as mass, length and energy, how to convert any value with unit to reduced/lj unit?

right now my solution is:

    def reduce(self, value: float, unit: str) -> float:

        origin = value * self.ureg(unit)
        base_unit = origin.dimensionality
        if "[length]" in base_unit:
            origin /= self.fdistance
        if "[mass]" in base_unit:
            origin /= self.fmass
        if "[energy]" in base_unit:
            origin /= self.fenergy
        if "[time]" in base_unit:
            origin *= self.ftime
        return origin.to("").magnitude

where unit startswith f is convert factor:

    def set_fundamental(self, mass: str, distance: str, energy: str):
        self.fmass = self.ureg(mass)
        self.fdistance = self.ureg(distance)
        self.fenergy = self.ureg(energy)
        self.ftime = (self.fenergy / (self.fmass * self.fdistance**2))**0.5

which follows the rule from https://docs.lammps.org/units.html

I think those are same question maybe with similar solution.

I dont have any idea how to do that