I dunno, I think contra's a terrible idea, and the sooner it dies the better. It's pretty much an abuse of the T+3 settlement system.
It's bad for the brokers, because they're handing out interest-free loans instead of being able to charge interest on margin loans. It's bad for the exchange, because it cripples the development of derivatives markets and stock lending (because you don't need to borrow stock to be able to short it via contra). And it's bad for investors, because it gives them infinite leverage and thus infinite capacity to blow themselves up.
Just to clear any misconception regarding shorting of stock, borrowing of share is required if the short position is held overnight regardless whether it is contra position or not.