# NOT RUN {
df <- data.frame(a = LETTERS[sample(5, 15, replace=TRUE)], y = rnorm(15))
combine_factor(df$a, c(1,2,2,1,2))
combine_factor(df$a, c(1:4, 1))
(f <- reorder(df$a, df$y))
percent <- tapply(abs(df$y), df$a, sum)
combine_factor(f, c(order(percent)[1:3]))
# }
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