Welcome to the library management system, please type your commands. 
> b1 added to the system.
> b2 added to the system.
> c1 added to the system.
> c2 added to the system.
> Success! b1 is borrowed by c1.
> Fail! The book was borrowed by another customer.
> Success! b2 is borrowed by c2.
> 1 b1 borrowed
2 b2 borrowed
> 1 c1
2 c2
> Success! The following book has been returned:
1 b1
> Success! b1 is borrowed by c2.
> Book borrowed by c2:
1 b1
2 b2
> Success! The following book has been returned:
1 b1
> Book borrowed by c2:
2 b2
> Success! The following book has been returned:
2 b2
> Fail! The book has not been borrowed.
> Exit the system!

Welcome to the library management system, please type your commands.
> b1 added to the system.
> b2 added to the system.
> c1 added to the system.
> c2 added to the system.
> Success! b1 is borrowed by c1.
> Fail! The book was borrowed by another customer.
> Success! b2 is borrowed by c2.
> 1 b1 borrowed
2 b2 borrowed
> 1 c1
2 c2
> Success! The following book has been returned:
1 b1
> Success! b1 is borrowed by c2.
> Book borrowed by c2:
1 b1
2 b2
> Success! The following book has been returned:
1 b1
> Book borrowed by c2:
2 b2
> Success! The following book has been returned:
2 b2
> Fail! The book has not been borrowed.
> Exit the system!