#' Determines the factor levels of a factor type vector
#' that are actually occuring in a given vector.
#'
#' @param x [\code{vector}]\cr
#' Source vector of factors.
#' @return [\code{character}]
#' Character vector of used levels.
#' @export
usedFactorLevels = function(x) {
stopifnot(is.factor(x))
return(intersect(levels(x), unique(x)))
}
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