code: func NewDeviceAllocator(lastIndex int) DeviceAllocator {
  	possibleDevices := []mountDevice{}
  	for _, firstChar := range []rune{'b', 'c'} {
  		for i := 'a'; i <= 'z'; i++ {
  			dev := mountDevice([]rune{firstChar, i})
  			possibleDevices = append(possibleDevices, dev)
  		}
  	}
  	return &deviceAllocator{
  		possibleDevices: possibleDevices,
  		lastIndex:       lastIndex,
  	}
  }
docs: // NewDeviceAllocator allocates device names according to scheme ba..bz, ca..cz
  // it moves along the ring and always picks next device until
  // device list is exhausted.