code: public boolean returnLoan(Loan loan) {
          if (loan != null) {
              Book book = loan.getBook();
              if (book != null) {
                  String isbn = book.getIsbn();
                  if (isbn != null) {
                      synchronized (librarian) {
                          Integer quantity = isbns_to_quantities.get(isbn);
                          if (quantity != null) {
                              quantity = new Integer(quantity.intValue() + 1);
                              isbns_to_quantities.put(isbn, quantity);
                              return true;
                          }
                      }
                  }
              }
          }
          return false;
      }
docs: Return loan.
  
  @param loan the loan
  @return true, if successful