private EvolutionStart<G, C> invalidate(final EvolutionStart<G, C> start) {
		return EvolutionStart.of(
			start.population().map(Phenotype::nullifyFitness),
			start.generation()
		);
	}
Nullifies the fitness values of the population, if requested. The
<em>nullification flag</em> is reset after this call. Two consecutive
calls of this method might lead to two different results.

@see #nullifyFitness()

@param start the evolution start object
@return the evolution start object with the nullified fitness values,
        if the nullification has been triggered
jenetics/src/main/java/io/jenetics/engine/FitnessNullifier.java