[R6RS] cyclic list exceptions

[dyb at cs.indiana.edu]

> I assume you're also advocating that these procedures conceptually
> operate on a copy of the input list, in the way you described Chez
> Scheme's implementations?

I don't know.  For most of the procedures, it doesn't matter unless the
implementation is multithreaded.  It can matter for map, since the mapped
procedure might mutate one of the argument lists after, say, cycle or
same-length checks have been performed.  It would be nice if an
implementor had to be careful to make sure that this couldn't make map
sick, but I'm not sure how to deal with this in the report, especially if
we want to make it practical for multithreaded systems.

Kent