Dplyr / tidyverse：rename_at` .funs`必须包含一个重命名函数，而不是4

I have the following test data:

我有以下测试数据:

library(tidyverse)

df <- tibble(
  g1 = c(1, 1, 2, 2, 2),
  g2 = c(a, a, a, b, b),
  a = sample(5),
  b = sample(5)
)
l