def _sub(a, b):
        return Fraction(a.numerator * b.denominator -
                        b.numerator * a.denominator,
                        a.denominator * b.denominator)
Adds a client_id to the list of completed tasks.
dataset/ETHPy150Open deanhiller/databus/webapp/play1.3.x/python/Lib/fractions.py/Fraction._sub--crossover with--dataset/ETHPy150Open google/grr/grr/lib/hunts/implementation.py/GRRHunt.MarkClientDone