Liquidity risk讲义第一页提及more-volatile market condition often have a negative effect on bid-ask spread；bid-ask spread is stable only when markets are stable；另外，老师讲到liquidity上升bid-ask spread下降。
逻辑关系没有搞清楚，不明白第一句话原因
例如，股灾时market volatile，但是bid-ask spread会更大吧
