第一步发现：
scatter mpg weight by (tru)
variable by not found
r(111);

第二步：采用常用方法安装
ssc describe by
  ssc describe: "by" not found at SSC, type search by
  (To find all packages at SSC that start with b, type ssc describe b)
  r(601);

####出现错误提示，输入ssc describe b 发现安装包里面的确没有by,和错误601

请大神指点迷津