code: func NewDeviceAllocator() DeviceAllocator {
  	possibleDevices := make(map[mountDevice]int)
  	for _, firstChar := range []rune{'b', 'c'} {
  		for i := 'a'; i <= 'z'; i++ {
  			dev := mountDevice([]rune{firstChar, i})
  			possibleDevices[dev] = 0
  		}
  	}
  	return &deviceAllocator{
  		possibleDevices: possibleDevices,
  		counter:         0,
  	}
  }
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.