memory increasing when generating lots of hyperelliptic curves (without storing them)

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Executing the code given below (it generates hyperelliptic curves), the used memory increases all the time. Since nothing is stored permanently, this should not happen. How do I avoid it? (It is not really a problem with the example given, but using more loops, it becomes a problem.)

Here an example code:

F = GF(4, 'a')
R.<t> = PolynomialRing(F)

for x in F:
    for y in F:
        for z in F:
            h = x*t^2 + y*t + z

            for a in F:
                for b in F:
                    for c in F:
                        for d in F:
                            for e in F:
                                f = t^5 + a*t^4 + b*t^3 + c*t^2 + d*t + e

                                C = HyperellipticCurve(f,h)