docs: @Override
  	public int getNumberLeaves() {
  		while (true) {
  			try {
  				int result = 0;
  				for (Slot slot: subSlots){
  					result += slot.getNumberLeaves();
  				}
  				return result;
  			}
  			catch (ConcurrentModificationException e) {
  				// ignore and retry
  			}
  		}
  	}