I have a function that can be used for summarizing a variable based on some user-defined groups, making use of `dplyr`:

```
library(tidyverse)

get_var_summary <- function(.data, .target_var, .group_vars = vars()) {
  .target_var = enquo(.target_var)
  return(
    .data %>%
      filter(!is.na(!! .target_var)) %>% 
      group_by_at(.vars = .group_vars) %>% 
      summarize(
        mean = mean(!! .target_var),
        sd = sd(!! .target_var),
        ci = qnorm(0.975) * sd(!! .target_var) / sqrt(n()),
        median = median(!! .target_var),
        n = n()
      ) %>% 
      mutate(
        sd = ifelse(is.na(sd), Inf, sd),
        ci = ifelse(is.na(ci), Inf, ci)
      ) %>% 
      ungroup()
  )
}

mtcars %>% 
  get_var_summary(wt, .group_vars = vars(cyl))

```

Returns:

```
# A tibble: 3 x 6
    cyl  mean    sd    ci median     n
  <dbl> <dbl> <dbl> <dbl>  <dbl> <int>
1    4.  2.29 0.570 0.337   2.20    11
2    6.  3.12 0.356 0.264   3.22     7
3    8.  4.00 0.759 0.398   3.76    14

```

Now, in order to be able to easily repeat the `.group_vars`, but occasionally supply another grouping var in addition, I would like to define another function that calls `get_var_summary`, but with one additional column added to `.group_vars`:

```
get_var_summary_by_another <- function(.data, .extra_var, .target_var, .group_vars = vars()) {

  # how do I add .extra_var to .group_vars?

}

```

How can I do that?