Library Book Reservation Queue

You are developing a library system where users can reserve books. When a book is reserved, users are placed in a queue until the book is available. Your system should handle the following commands: 1. `RESERVE USERNAME` - A user with `USERNAME` reserves a book. 2. `FILL` - Fill the reservation for the user at the front of the queue and print their username. If the queue is empty, print 'No reservations to fill.' 3. `STATUS` - Output the number of users currently waiting in the reservation queue. 4. `NEXT` - Print the username of the next user in line without removing them from the queue. If the queue is empty, print 'No users in queue.' Example Input: RESERVE Alice RESERVE Bob STATUS FILL STATUS NEXT FILL FILL Example Output: 2 Alice 1 No reservations to fill.