public Generation evolve() {
	Set<Cell> newGeneration = new HashSet<Cell>();
	allCandidateToBeAliveInTheNextGeneration().forEach((Cell candidate) -> {
		int aliveNeighbors = candidate.neighbors().count((Cell c) -> isAlive(c));
		State candidateStateInNewGeneration = rules.nextState(state(candidate), aliveNeighbors);
		if (candidateStateInNewGeneration == State.ALIVE) {
			newGeneration.add(candidate);
		}
	});
	return new Generation(newGeneration);
}

private Zone allCandidateToBeAliveInTheNextGeneration() {
	Zone toCalculate = aliveCells
		.stream()
		.map((alive) -> alive.block())
		.reduce(
			Zone.empty(),
			(Zone z1, Zone z2) -> z1.union(z2)
		);
	return toCalculate;
}