Better syntax for mutate() than nested "ifelse" functions

[reedblog]

All of the dplyr functions make R code more readable except for one pretty common case, conditionally assigning variable values with mutate(). The current syntax is something like:
df <- mutate(df, newvar = ifelse(oldvar %in% c("a", "b", "c"), "group1", ifelse(oldvar %in% c("d", "e", "f"), "group2", ifelse(oldvar %in% c("g", "h", "i"), "group3", NA))))

I saw a question on StackOverflow that gave a much better solution, borrowing from the "mosaic" package: http://stackoverflow.com/questions/24459752/can-dplyr-package-be-used-for-conditional-mutating?rq=1

df <- mutate(df, newvar = derivedFactor(
  "group1" = oldvar %in% c("a", "b", "c"),
  "group2" = oldvar %in% c("d", "e", "f"),
  "group3" = oldvar %in% c("g", "h", "i"),
  .default = NA
))

Could something like this be the default syntax, please?

Data frame for testing: df <- data.frame(oldvar = letters)

[romainfrancois]

Duplicate of #631