可以参见：

    07. 环境

    08. R 的面向对象

    我们可以通过:: 访问包中的函数和内容（必须得是export 给使用者的），如果想要访问全部的内容，可以使用::: 。

    对于不确定来源的对象（包括方法），我们可以使用：

    1. > getAnywhere(x)
    2. A single object matching 'x' was found
    3. It was found in the following places
    4.   .GlobalEnv
    5. with value
    7.  [1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19
    8. > getAnywhere(read.maf)
    9. no object named 'read.maf' was found
    10. > getAnywhere(summary)
    11. 2 differing objects matching 'summary' were found
    12. in the following places
    13.   package:base
    14.   namespace:DBI
    15.   namespace:base
    16. Use [] to view one of them
    17. > getAnywhere(summary)[1]
    18. function (object, ...) 
    19. UseMethod("summary")
    20. <bytecode: 0x7f9429c59c70>
    21. <environment: namespace:base>