code: public void addHeuristicDiskCost(double cost) {
  		if (cost <= 0) {
  			throw new IllegalArgumentException("Heuristic costs must be positive.");
  		}
  		this.heuristicDiskCost += cost;
  		// check for overflow
  		if (this.heuristicDiskCost < 0) {
  			this.heuristicDiskCost = Double.MAX_VALUE;
  		}
  	}
docs: Adds the heuristic costs for disk to the current heuristic disk costs
  for this Costs object.
  
  @param cost The heuristic disk cost to add.