Puzzled about a new method for "[".

You can try for testing with a column of class errors, from the package 
'errors'. The attributes depend on the content in the way Hadley pointed 
out.

Thanks, but it turns out to be much simpler than that.  There is a very 
easy way to accomplish what I want --- simply give an attribute to the 
data frame, rather than to a certain column of that data frame.  I don't 
know why the hell I didn't do that in the first place!  Duh!!!

Sorry for all the noise that this issue has generated.

cheers,

Rolf

Honorary Research Fellow
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276