does subset.data.frame need to accept extra arguments?
It needs to have ... in the formal argument list because the generic subset() does. It could enforce a run-time warning that some arguments were being skipped (by testing length(list(...)) for example), but then NextMethod might fail, in a case where an object has a complicated class vector.
That wouldn't be a problem if the check was implemented in subset.data.frame, would it? Hadley -- Chief Scientist, RStudio http://had.co.nz/