int nbFallenLeaves = 20;
Set<TreeLeafState> fallenLeaves = new HashSet<TreeLeafState>(nbFallenLeaves);
for (int i = 0; i < nbFallenLeaves; i++) {
fallenLeaves.add(createSpecificTreeLeafState());
}
return new UniverseState(age, fpsShowing, trees, fallenLeaves, createSpecificEnvironmentState(),
createSpecificTimeCounterState());
}