public void Undo()
 {
     if (_prevCommand == _fan.HIGH)
     {
         _fan.High();
     }
     else if (_prevCommand == _fan.MEDIUM)
     {
         _fan.Medium();
     }
     else if (_prevCommand == _fan.LOW)
     {
         _fan.Low();
     }
     else if (_prevCommand == _fan.OFF)
     {
         _fan.Off();
     }
 }
 public void Execute()
 {
     _prevCommand = _fan.Speed;
     _fan.High();
 }