public void Visit(Shield item)
 {
     Console.WriteLine($"\n{nameof(Shield)} received an upgrade!");
     item.Defense += 10;
     item.Speed   += 5;
 }
 public void Visit(Shield item)
 {
     Console.WriteLine($"\n{nameof(Shield)} suffered a downgrade!");
     item.Defense -= 5;
     item.Speed   -= 3;
 }