Add a "Total" level to each specified categorical variable in the data frame

df.add.total.category  <- function(dat, var.list, new.level.label = "Total")
{ 
    ## Purpose: Add a "Total" level to each specified categorical variable in the data frame. 
    ## Arguments:
    ##   dat: a data frame with several categorical variables for making tables.
    ##   var.list: a list of variable names in "dat".
    ##   new.level.label: the name for the new "total" level for the specified variable. 
    ## Return: 
    ## Author: Feiming Chen, Date: 21 Jun 2017, 13:02
    ## ________________________________________________

    if ( missing(var.list) )  {
        var.list <- names(dat)
    }

    d0 <- dat                           # to make new label across all variables
    lapply(var.list,  function( v )  {
        d <- dat
        d[[v]] <- new.level.label
        d0[[v]] <<- new.level.label
        d
    }) -> dd

    do.call(rbind, c(list(dat), dd, list(d0)))
}
if (F) {                                # Unit Test
    dat <- data.frame(A=gl(3,3), B=gl(3, 1, 9), C=11:19)
    df.add.total.category(dat, var.list=c("A", "B"))
}