Add additional levels to a factor

## Arguments

- f
A factor (or character vector).

- ...
Additional levels to add to the factor. Levels that already exist will be silently ignored.

- after
Where should the new values be placed?

## See also

`fct_drop()`

to drop unused factor levels.

## Examples

```
f <- factor(sample(letters[1:3], 20, replace = TRUE))
f
#> [1] c a b a b b a c c b b b b c c c a b b c
#> Levels: a b c
fct_expand(f, "d", "e", "f")
#> [1] c a b a b b a c c b b b b c c c a b b c
#> Levels: a b c d e f
fct_expand(f, letters[1:6])
#> [1] c a b a b b a c c b b b b c c c a b b c
#> Levels: a b c d e f
fct_expand(f, "Z", after = 0)
#> [1] c a b a b b a c c b b b b c c c a b b c
#> Levels: Z a b c
```