[plt-scheme] Mutable and immutable pairs in PLT Scheme

>> these are the attitudes that make PLT Scheme so cool, imhumbleiamnobodyo.
> Just to check: is this sarcastic? Assuming so (and I apologize in
> advance if I mistake your intent)

okeydokey, apology accepted. :-) my own apologies for apparently
coming across very poorly. i am not somebody who contributes to or
makes use of PLT a lot compared to the real movers and shakers, hence
my 'humble' 'nobody' status. but my personal gut knee jerk feeling
based on experience is that immutable is way better whenever possible.

sincerely.