docs: public long sum() {
  		long sum = base;
  		Cell[] as = cells;
  		if (as != null) {
  			int n = as.length;
  			for (int i = 0; i < n; ++i) {
  				Cell a = as[i];
  				if (a != null) sum += a.value;
  			}
  		}
  		return sum;
  	}