> mydf <- data.frame(date = as.Date("2010-01-01") + sample(500, size = 20),
> subject = sample(c("A", "B", "C"), 20, replace = TRUE), first = FALSE)
> mydf <- mydf[order(mydf$date),]
> df_split <- split(mydf, f = mydf$subject)
> df_split <- lapply(df_split, function(df) {df[1, "first"] <- TRUE; df})
> mydf <- do.call(rbind, df_split)
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