class Power(float):
    phys_unit = 'Watt'
    formal_letter = 'P'
    abbreviation = 'W'

    def __mul__(self, other):
        pass

    def __div__(self, other):
        if isinstance(other, Voltage):
            return Current(self / other)

        elif isinstance(other, Current):
            return Voltage(self / other)


class Resistance(float):
    phys_unit = 'Ohm'
    formal_letter = 'R'
    # abbreviation = 'Ω'

    def __mul__(self, other):
        if isinstance(other, Current):
            return Voltage(self * other)

    def __div__(self, other):
        pass


class Current(float):
    phys_unit = 'Ampere'
    formal_letter = 'I'
    abbreviation = 'A'

    def __mul__(self, other):
        if isinstance(other, Voltage):
            return Power(self * other)

    def __div__(self, other):
        pass

class Voltage(float):
    phys_unit = 'Volt'
    formal_letter = 'U'
    abbreviation = 'V'

    def __mul__(self, other):
        if isinstance(other, Current):
            return Power(self * other)


    def __div__(self, other):
        if isinstance(other, Current):
            return Resistance(self / other)

        elif isinstance(other, Resistance):
            return Current(self / other)