See this example in Odersky book http://books.google.ca/books?id=MFjNhTjeQKkC&pg=PA566&lpg=PA566&dq=scala+hashcode+41&source=bl&ots=FLri-FFRqv&sig=Q33MshJghSMfvg4dRuombaOwVms&hl=en&sa=X&ei=0OpaUeGZEe7U0gHt84HQBw&ved=0CHYQ6AEwCQ#v=onepage&q=scala%20hashcode%2041&f=false