layers[nbC - 1] = new BitSet(nbS);
layers[nbC - 1].set(nbS - 1, true);
available.set(nbS - 1, false);
for (int i = available.nextSetBit(0); i >= 0; i = available.nextSetBit(i + 1)) {
int layer = 1 + rand.nextInt(nbC - 2);
layers[layer].set(i, true);
available.set(i, false);
}