Arguments to `new_equations()` should enumerate possible options in the function's signature

[maurolepore]

• Enumerate possible options (see https://design.tidyverse.org/def-enum.html). If the argument x to f() can take "a", "b", and "c", then f() should be:

  
  f <- function(x = c("a", "b", "c")) {
  ...
  }