rapply() 中的参数how = c("unlist", "replace", "list")，list和unlist是分别以列表和非列表的形式输出结果，replace呢？有人知道吗，求指点，R的帮助看不太懂啊
If how = "replace", each element of the list which is not itself a list and has a class included in classes is replaced by the result of applying f to the element.
If the mode is how = "list" or how = "unlist", the list is copied, all non-list elements which have a class included in classes are replaced by the result of applying f to the element and all others are replaced by deflt. Finally, if how = "unlist", unlist(recursive = TRUE) is called on the result.