public void setStart(MazeCell maze_cell) {
   this.start = maze_cell;
   if (maze_cell.isBlocked()) maze_cell.setCost(1);
 }
 public void setGoal(MazeCell maze_cell) {
   this.goal = maze_cell;
   if (maze_cell.isBlocked()) maze_cell.setCost(1);
 }