Date: Fri, 18 Sep 2009 15:51:57 -0400
   From: David Van Horn <xxxxxx@cs.brandeis.edu>

   In a system that uses random-access lists everywhere, you would not need
   conversions.  In a system that doesn't, these are easy to portably
   define.  So now I'm not sure what your concern is.

I want to be able to write portable programs that can take advantage
of systems with only random-access lists, but that can run on other
systems as well by converting between sequential lists and random-
access lists.  `Take advantage' here means using identity conversions.
Of course I can define copying conversions portably and use those, but
that wouldn't take advantage of systems whose lists are all
random-access.