be up动词+介词
To hit a person when he’s down would be right up the double-dealer’s alley.（落井下石是那个两面派的拿手好戏。）be up one’s alley可以直译为对某人的路子，表示是某人所长，合某人的胃口。
be with动词+介词
I was with you as far as your last point, and then I got confused.（你说到最后一点时我还能理解，再往后就糊涂了。）be with一般表示同意或喜欢某事，但这里只表示理解某事。
