base::droplevels(), does not drop
NA levels that have values.
A factor (or character vector).
A character vector restricting the set of levels to be dropped. If supplied, only levels that have no entries and appear in this vector will be removed.
fct_expand() to add additional levels to a factor.
#>  a b #> Levels: a b cfct_drop(f)#>  a b #> Levels: a b# Set only to restrict which levels to drop fct_drop(f, only = "a")#>  a b #> Levels: a b cfct_drop(f, only = "c")#>  a b #> Levels: a b