大家来分析一下这句话中介词短语作什么成分
It is difficult to the point of impossibility for the average reader under the age of forty to image a time when high-quality arts criticism could be found in most big-city newspapers.
其中it是作形式主语而后面的to do作实际主语,我不清楚的是for这个介词短语作什么成分,是定语还是宾补,还有它前面那个to介词短语.