public boolean nullifyFitness() {
		return !_invalid.getAndSet(true);
	}
Triggers the nullification of the fitness values of the population for
the next generation.

@see #before(EvolutionStart)

@return {@code true} if the <em>nullification</em> request will trigger
        a new fitness nullification. @{code false} if the fitness
        <em>nullification</em> has been requested before, without
        actually executing it.
jenetics/src/main/java/io/jenetics/engine/FitnessNullifier.java