Grim tidings

For every seller there’s supposed to be a buyer. But why is anyone buying Ambac’s stock now? The bond insurer’s shares plummeted nearly 40% after it revealed its latest big loss. Its book of credit derivatives is plunging in value, and it’s doing little new business. Even boss Michael Callen admits there’s no way to know if the worst is past. There are reasons for optimism. First, Ambac’s liquidity isn’t under immediate threat. Its credit default swaps don’t require it to post collateral, so it...