we have a book, and there many people want to borrow that book
and the book can only be took by one person.
the variable "available" means that the book is sent back and can be borrowed again.

// for each person that wants to borrow a book
while(true)
{
wait(available); // wait until the book is available
//critical section
signal(available); //give an available signal that the book can be borrow
//remainder section
}