This is a useful way of patching together factors from multiple sources that really should have the same levels but don't.
#>  1 1 1 2fct_c(fa, fb, fab)#>  a b a b #> Levels: a b# You can also pass a list of factors with !!! fs <- list(fa, fb, fab) fct_c(!!!fs)#>  a b a b #> Levels: a b