code: public Collection<Book> getAvailableBooks() {
          synchronized (librarian) {
              Collection<Book> books = new LinkedList<Book>();
              for (Entry<String, Integer> entry : isbns_to_quantities.entrySet()) {
                  if (entry.getValue() > 0) {
                      books.add(getBook(entry.getKey()));
                  }
              }
              return books;
          }
      }
docs: Gets the available books.
  
  @return the available books