5.3 连词if的省略
如果虚拟条件句的谓语部分有were，had和should时，可省略if，把were，had和should放到从句主语前面去，多见于书面语。
Had you not helped me，I should have failed.
要是没有你的帮助，我就失败了。
Were you in my position, you would do the same.
假如你处在我的地位，你也会这样干的。
Should they attack us, we’ll wipe them out completely.
假如他们进攻我们，我们就把他们彻底消灭干净。
